lessen (v.) Look up lessen at Dictionary.com
"to become less," c. 1300, from less + -en (1). Related: Lessened; lessening.
lesser (adj.) Look up lesser at Dictionary.com
early 13c., a double comparative, from less + -er (2). Johnson calls it "a barbarous corruption of less, formed by the vulgar from the habit of terminating comparatives in -er." As an adverb from 1590s; now generally poetic or obsolete except in expression lesser-known (1813).
lesson (n.) Look up lesson at Dictionary.com
early 13c., "a reading aloud from the Bible," also "something to be learned by a student," from Old French leçon, from Latin lectionem (nominative lectio) "a reading," noun of action from past participle stem of legere "to read" (see lecture (n.)). Transferred sense of "an occurrence from which something can be learned" is from 1580s.
lessor (n.) Look up lessor at Dictionary.com
"one who grants a lease," late 14c., from Anglo-French lessor (late 13c.), from verb lesser (see lease).
lest (conj.) Look up lest at Dictionary.com
c. 1200, contracted from Middle English phrase les te "less that," from Old English phrase þy læs þe "whereby less that," from þy, instrumental case of demonstrative article þæt "that" + læs (see less) + þe "the." The þy was dropped and the remaining two words contracted into leste.
let (v.) Look up let at Dictionary.com
Old English lætan "to allow to remain; let go, leave, depart from; leave undone; to allow; bequeath," also "to rent" (class VII strong verb; past tense let, past participle læten), from Proto-Germanic *letan (cognates: Old Saxon latan, Old Frisian leta, Dutch laten, German lassen, Gothic letan "to leave, let"), from PIE *le- (2) "to let go, slacken" (cognates: Latin lassus "faint, weary," Lithuanian leisti "to let, to let loose;" see lenient). If that derivation is correct, the primary sense would be "let go through weariness, neglect."

Of blood, from late Old English. To let (something) slip originally (1520s) was a reference to hounds on a leash; figurative use from 1540s. To let (someone) off "allow to go unpunished" is from 1814. To let on "reveal, divulge" is from 1725; to let up "cease, stop" is from 1787. Let alone "not to mention" is from 1812.
let (n.) Look up let at Dictionary.com
"stoppage, obstruction" (obsolete unless in legal contracts), late 12c., from archaic verb letten "to hinder," from Old English lettan "hinder, delay," from Proto-Germanic *latjan (cognates: Old Saxon lettian "to hinder," Old Norse letja "to hold back," Old High German lezzen "to stop, check," Gothic latjan "to hinder, make late," Old English læt "sluggish, slow, late"); see late.
let up (n.) Look up let up at Dictionary.com
"cessation," 1837, from verbal phrase let up "cease, stop" (1787). In Old English the phrase meant "to put ashore."
letch (n.) Look up letch at Dictionary.com
"craving, longing," 1796, perhaps a back-formation from lecher, or from a figurative use of latch (v.) in a secondary sense of "grasp, grasp on to."
letdown (n.) Look up letdown at Dictionary.com
also let-down, "disappointment," 1768, from let (v.) + down (adv.). The verbal phrase is from mid-12c. in a literal sense; figuratively by 1795.
lethal (adj.) Look up lethal at Dictionary.com
1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from letum "death," of uncertain origin. Form altered in Late Latin by association with lethes hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness."
lethality (n.) Look up lethality at Dictionary.com
1650s, from lethal + -ity.
lethargic (adj.) Look up lethargic at Dictionary.com
late 14c., litargik, from Latin lethargicus "affected with lethargy," from Greek lethargikos, from lethargos (see lethargy). Related: Lethargically.
lethargy (n.) Look up lethargy at Dictionary.com
late 14c., litarge, from Old French litargie or directly from Medieval Latin litargia, from Late Latin lethargia, from Greek lethargia "forgetfulness," from lethargos "forgetful," originally "inactive through forgetfulness," from lethe "forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English.
Lethe Look up Lethe at Dictionary.com
river of Hades (whose water when drunk caused forgetfulness of the past), from Greek lethe, literally "forgetfulness, oblivion," related to lethargos "forgetful," lathre "secretly, by stealth," lathrios "stealthy," lanthanein "to be hidden." Cognate with Latin latere "to be hidden" (see latent). Related: Lethean.
Letitia Look up Letitia at Dictionary.com
fem. proper name, literally "gladness," from Latin laetitia, from laetus "glad," of unknown origin.
Lett Look up Lett at Dictionary.com
1831, from German Lette, from Old High German liuti "people" (German Leute). The native name is Latvji (see Latvia). Related: Lettic; Lettish.
letter (n.1) Look up letter at Dictionary.com
c. 1200, "graphic symbol, alphabetic sign, written character," from Old French letre (10c., Modern French lettre) "character, letter; missive, note," in plural, "literature, writing, learning," from Latin littera (also litera) "letter of the alphabet," of uncertain origin, perhaps via Etruscan from Greek diphthera "tablet," with change of d- to l- as in lachrymose. In this sense it replaced Old English bocstæf, literally "book staff" (compare German Buchstabe "letter, character," from Old High German buohstab, from Proto-Germanic *bok-staba-m).

Latin littera also meant "a writing, document, record," and in plural litteræ "a letter, epistle," a sense first attested in English early 13c., replacing Old English ærendgewrit, literally "errand-writing." The Latin plural also meant "literature, books," and figuratively "learning, liberal education, schooling" (see letters). School letter in sports, attested by 1908, were said to have been first awarded by University of Chicago football coach Amos Alonzo Stagg. Expression to the letter "precisely" is from 1520s (earlier as after the letter). Letter-perfect is from 1845, originally in theater jargon, in reference to an actor knowing the lines exactly. Letter-press, in reference to matter printed from relief surfaces, is from 1840.
letter (n.2) Look up letter at Dictionary.com
"one who lets" in any sense, c. 1400, agent noun from let (v.).
letter (v.) Look up letter at Dictionary.com
"to write in letters," 1660s, from letter (n.1). Earlier it meant "to instruct" (mid-15c.). Related: Lettered; lettering.
lettered (adj.) Look up lettered at Dictionary.com
"literate," c. 1300, from letter (n.). Meaning "inscribed" is from 1660s.
letterhead (n.) Look up letterhead at Dictionary.com
1868, short for letterheading (1867); from letter (n.1) + head (n.). So called because it was printed at the "head" of the piece of paper.
lettering (n.) Look up lettering at Dictionary.com
1640s, "act of writing;" 1811 as "act of putting letters on something," verbal noun from letter (v.).
letters (n.) Look up letters at Dictionary.com
"the profession of authorship or literature," mid-13c., from plural of letter (n.).
lettuce (n.) Look up lettuce at Dictionary.com
late 13c., probably from Old French laitues, plural of laitue "lettuce," from Latin lactuca "lettuce," from lac (genitive lactis) "milk" (see lacto-); so called for the milky juice of the plant.
leu (n.) Look up leu at Dictionary.com
monetary unit of Romania, introduced 1867, literally "lion." Monetary names in the Balkans often translate as "lion" because Dutch gold coins stamped with lions circulated widely in the region in the 17c. and the word for "lion" came to be a word for "money" in some languages in the region.
leukaemia (n.) Look up leukaemia at Dictionary.com
alternative spelling of leukemia.
leukemia (n.) Look up leukemia at Dictionary.com
1851, on model of German Leukämie (1848), coined by R. Virchow from Greek leukos "clear, white" (cognate with Gothic liuhaþ, Old English leoht "light;" see light (n.)) + haima "blood" (see -emia).
leukemic (adj.) Look up leukemic at Dictionary.com
also leukaemic, 1852; see leukemia + -ic.
leukocyte (n.) Look up leukocyte at Dictionary.com
also leucocyte, 1860, via French leucocyte, from Greek leuko-, comb. form of leukos "white" (see light (n.)) + -cyte (see cyto-).
lev (n.) Look up lev at Dictionary.com
monetary unit of Bulgaria, introduced 1881, literally "lion" (see leu).
Levant Look up Levant at Dictionary.com
"Mediterranean lands east of Italy," late 15c., from Middle French levant "the Orient," from present participle of lever "to rise" (from Latin levare "to raise;" see lever). The region so called in reference to the direction of sunrise.
Levantine (adj.) Look up Levantine at Dictionary.com
1640s, from Levant + -ine (1).
levari facias Look up levari facias at Dictionary.com
Latin, literally "cause to be levied."
levator (n.) Look up levator at Dictionary.com
from medical Latin levator "a lifter," from Latin levatus, past participle of levare "to raise" (see lever).
levee (n.1) Look up levee at Dictionary.com
1719, "natural or artificial embankment to prevent overflow of a river," from New Orleans French levée "raising, lifting; embankment," from French, originally fem. past participle of lever "to raise," from Latin levare "to raise" (see lever).
levee (n.2) Look up levee at Dictionary.com
"morning assembly held by a prince or king (upon rising from bed)," 1670s, from French lever "a raising," noun use of verb meaning "to raise" (see levee (n.1)).
level (n.) Look up level at Dictionary.com
mid-14c., "tool to indicate a horizontal line," from Old French livel "a level" (13c.), ultimately from Latin libella "a balance, level," diminutive of libra "balance, scale, unit of weight," from PIE *lithra- "a scale." Cognate Spanish nivel, Modern French niveau are from the same source but altered by dissimilation. Meaning "horizontality" is from c. 1400. Meaning "position as marked by a horizontal line" is from 1530s. Phrase on the level "fair, honest" is from 1872; earlier it meant "moderate, without great ambition" (1790).
level (adj.) Look up level at Dictionary.com
early 15c., from level (n.). To do one's level best is from 1851.
level (v.) Look up level at Dictionary.com
mid-15c., "to make level," from level (n.). From c. 1600 as "to bring to a level;" 1958 as "to cease increasing." Meaning "to aim a gun" is late 15c. Slang sense of "tell the truth" is from 1920. To level up "to rise" is attested by 1863.
A word here as to the misconception labored under by our English neighbor; he evidently does not understand the American manner of doing things. We never level down in this country; we are always at work on the up grade. "Level up! Level up!" is the motto of the American people. [James E. Garretson, "Professional Education," in "The Dental Cosmos," Philadelphia, 1865]
To level off "cease rising or falling" is from 1920, originally in aviation.
levelheaded (adj.) Look up levelheaded at Dictionary.com
also level-headed, 1869, from level (adj.) + head (n.). The notion is of "balanced." Related: Levelheadedness.
leveller (n.) Look up leveller at Dictionary.com
1590s, someone or something that makes level; agent noun of level (v.). From 1640s as the name of a political party of the time of Charles I that advocated abolishing all differences of position and rank.
lever (n.) Look up lever at Dictionary.com
c. 1300, from Old French levier (Modern French leveur) "a lifter, a lever," agent noun from lever "to raise," from Latin levare "to raise," from levis "light" in weight, from PIE root *legwh- "light, having little weight; easy, agile, nimble" (cognates: Sanskrit laghuh "quick, small;" Greek elakhys "small," elaphros "light;" Old Church Slavonic liguku, Lithuanian lengvas "light;" Old Irish laigiu "smaller, worse;" Gothic leihts, Old English leoht "light" (adj.)). As a verb, 1856, from the noun.
leverage (n.) Look up leverage at Dictionary.com
1724, "action of a lever," from lever (n.) + -age. Meaning "power or force of a lever" is from 1827; figurative sense from 1858. The financial sense is attested by 1933, American English; as a verb by 1956. Related: Leveraged; leverages; leveraging.
leveret (n.) Look up leveret at Dictionary.com
"young hare," early 15c., from Old French levrat, diminutive of levre (12c., Modern French lièvre) "hare," from Latin lepore, from lepus.
Levi Look up Levi at Dictionary.com
masc. proper name, biblical son of Jacob by Leah, from Hebrew lewi, literally "joining, pledging, attached," from stem of lawah "he joined."
leviathan (n.) Look up leviathan at Dictionary.com
late 14c., "sea monster, sea serpent," also regarded as a form of Satan, from Late Latin leviathan, from Hebrew livyathan "dragon, serpent, huge sea animal," of unknown origin, perhaps related to liwyah "wreath," from root l-w-h- "to wind, turn, twist." Of powerful persons or things from c. 1600. Hobbes's use is from 1651.
levirate (n.) Look up levirate at Dictionary.com
custom by which the male next-of-kin of a dead man was bound to marry his widow, 1725, from Latin levir "brother-in-law" (from PIE *daiwer- "husband's brother") + -ate (2).
Levis (n.) Look up Levis at Dictionary.com
1926, American English, originally Levi's, from Levi Strauss and Company, original manufacturer. Strauss' innovation was the copper rivets at strain points. A cowboy's accessory, adopted as a fashion c. 1940s.
levitate (v.) Look up levitate at Dictionary.com
1670s, "to rise by virtue of lightness," from Latin levitas "lightness," patterned in English on gravitate. Sense of "raise (a person) into the air" is mainly from spiritualism (1870s). Related: Levitated; levitating.