undemonstrative (adj.) Look up undemonstrative at Dictionary.com
1826, "not characterized by outward expression of feelings," from un- (1) "not" + demonstrative (adj.).
undeniable (adj.) Look up undeniable at Dictionary.com
1540s, from un- (1) "not" + deny + -able. In 19c., often with a sense of "undeniably good." Related: Undeniably.
under (prep., adv.) Look up under at Dictionary.com
Old English under (prep.) "beneath, among, before, in the presence of, in subjection to, under the rule of, by means of," also, as an adverb, "beneath, below, underneath," expressing position with reference to that which is above, from Proto-Germanic *under- (cognates: Old Frisian under, Dutch onder, Old High German untar, German unter, Old Norse undir, Gothic undar), from PIE *ndher- "under" (cognates: Sanskrit adhah "below;" Avestan athara- "lower;" Latin infernus "lower," infra "below").

Productive as a prefix in Old English, as in German and Scandinavian (often forming words modeled on Latin ones in sub-). Notion of "inferior in rank, position, etc." was present in Old English. With reference to standards, "less than in age, price, value," etc., late 14c. As an adjective, "lower in position; lower in rank or degree" from 13c. Also used in Old English as a preposition meaning "between, among," as still in under these circumstances, etc. (though this may be an entirely separate root; see understand).

Under the weather "indisposed" is from 1810. Under the table is from 1921 in the sense of "very drunk," 1940s in sense of "illegal." To get something under (one's) belt is from 1954; to keep something under (one's) hat "secret" is from 1885; to have something under (one's) nose "in plain sight" is from 1540s; to speak under (one's) breath "in a low voice" is attested from 1832. To be under (someone's) thumb "entirely controlled" (by that person) is recorded from 1754.
under-achiever (n.) Look up under-achiever at Dictionary.com
also underachiever, 1953, from under + agent noun from achieve (v.). Under-achievement is recorded from 1951.
under-employed (adj.) Look up under-employed at Dictionary.com
1908, "not used to optimum capacity," originally in reference to working persons, from under + past participle of employ (v.).
under-employment (n.) Look up under-employment at Dictionary.com
also underemployment, 1909, from under + employment.
underage (adj.) Look up underage at Dictionary.com
also under-age, 1590s, from under + age (n.).
underarm (adj.) Look up underarm at Dictionary.com
1816, "underhand" (in reference to a style of throwing), from under + arm (n.1). First attested 1908 in dressmaking sense of "seams on the lower half of the arm-hole;" as a euphemism for armpit, it is attested from 1930s, popularized by advertisers.
underbelly (n.) Look up underbelly at Dictionary.com
c.1600, from under + belly (n.). In figurative sense of "most vulnerable part" it is recorded from Churchill's 1942 speech. Sometimes used erroneously or euphemistically in sense of "seamy or sordid part" of anything.
underbid (v.) Look up underbid at Dictionary.com
1610s, from under + bid (v.). Related: Underbidding.
underbred (adj.) Look up underbred at Dictionary.com
"of inferior breeding, vulgar," 1640s, from under + past participle of breed (v.). Of animals, "not pure bred," attested from 1890.
underbrush (n.) Look up underbrush at Dictionary.com
"shrub and small trees in a forest," 1775, from under + brush (n.2). Originally American English; compare undergrowth, attested in the same sense from 1600.
undercarriage (n.) Look up undercarriage at Dictionary.com
1794, from under + carriage (n.). Meaning "landing gear of an aircraft" is recorded from 1911.
undercharge (v.) Look up undercharge at Dictionary.com
1630s, from under + charge (v.).
underclass (n.) Look up underclass at Dictionary.com
"subordinate social class," 1894, from under (adj.) + class (n.). A loan-translation of Swedish underklass.
underclassman (n.) Look up underclassman at Dictionary.com
"sophomore or freshman," 1869, American English, from under (adj.) + class (n.) in the school form sense + man (n.).
undercover (adj.) Look up undercover at Dictionary.com
1854, "sheltered," from under + cover (n.). Sense of "operating secretly" attested from 1920.
undercroft (n.) Look up undercroft at Dictionary.com
"crypt of a church; underground vault," late 14c., from under + croft.
undercurrent (n.) Look up undercurrent at Dictionary.com
1660s, "stream of water or air flowing beneath the surface or beneath another current," a hybrid formed from under + current (n.). The figurative sense of "suppressed or underlying character" is attested from 1817.
undercut (v.) Look up undercut at Dictionary.com
late 14c., "to cut down or off," from under + cut (v.). In the commercial sense of "sell at lower prices" (or work at lower wages) it is first attested 1884. Figurative sense of "render unstable, undermine" is recorded from 1955, from earlier literal meaning "cut so as to leave the upper portion larger than the lower" (1874).
underdeveloped (adj.) Look up underdeveloped at Dictionary.com
1892, in photography, from under + past participle of develop (v.). In reference to countries or regions, recorded from 1949.
underdog (n.) Look up underdog at Dictionary.com
"the beaten dog in a fight," 1887, from under + dog (n.). Compare top dog "dominant person in a situation or hierarchy" (see top (adj.)).
I'm a poor underdog
But tonight I will bark
With the great Overdog
That romps through the dark.

[from "Canis Major," Robert Frost, 1928]
underdone (adj.) Look up underdone at Dictionary.com
1680s, in reference to cooked meat, from under + done. Old English underdon (v.), Middle English underdo meant "to put under, to subject, subjugate."
underdressed (adj.) Look up underdressed at Dictionary.com
also under-dressed, "too plainly dressed," 1759, from under (adv.) + past participle of dress (v.).
underestimate (v.) Look up underestimate at Dictionary.com
1812, "to estimate at too low an amount," from under + estimate (v.). Meaning "to rank too low, undervalue" is recorded from 1850. Related: Underestimated; underestimating.
underexposed (adj.) Look up underexposed at Dictionary.com
1861, in photography, from under + past participle of expose (v.).
underfeed (v.) Look up underfeed at Dictionary.com
1650s, from under + feed (v.). Related: Underfed; underfeeding.
underfoot (adv.) Look up underfoot at Dictionary.com
c.1200, underfot "under the feet," from under + foot (n.). Compare similarly formed Middle Dutch ondervoete. As an adjective, attested from 1590s; in reference to persons, "continually in the way," it is recorded from 1891.
undergarment (n.) Look up undergarment at Dictionary.com
1520s, from under + garment (n.).
undergird (v.) Look up undergird at Dictionary.com
1520s, from under + gird (v.). Related: Undergirded; undergirding.
undergo (v.) Look up undergo at Dictionary.com
Old English undergan "obtain, get; undertake," from under + gan (see go (v.)). Compare similarly formed Middle Dutch ondergaen, Old High German untarkun, German untergehen, Danish undergaa. Sense of "submit to, endure" is attested from c.1300. Meaning "to pass through" (an alteration, etc.) is attested from 1630s. Related: Undergone; underwent.
undergrad (n.) Look up undergrad at Dictionary.com
short for undergraduate, 1827.
undergraduate (n.) Look up undergraduate at Dictionary.com
1620s, a hybrid formed from under + graduate (n.). British used fem. form undergraduette in 1920s-30s. As an adjective, in the school sense, from 1680s.
underground (adv.) Look up underground at Dictionary.com
1570s, "below the surface," from under + ground (n.). As an adjective, attested from c.1600; figurative sense of "hidden, secret" is attested from 1630s; adjectival meaning "subculture" is from 1953, from adjectival use in reference to World War II resistance movements against German occupation, on analogy of the dominant culture and the Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).
Underground Railroad (n.) Look up Underground Railroad at Dictionary.com
"network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.
undergrowth (n.) Look up undergrowth at Dictionary.com
"shrubs or small trees growing amid larger ones," c.1600, from under + growth.
underhand (adv.) Look up underhand at Dictionary.com
mid-14c., "by secret means, stealthily, in a surreptitious manner," from under + hand (n.). Perhaps the notion is of the hand turned over (thus concealing what it holds). Compare Middle Dutch onderhanden "by degrees, slowly," Dutch onderhandsch "secret, private." The adjective is attested from 1540s. Old English under hand meant "in subjection, in (one's) control or power."
underhanded (adj.) Look up underhanded at Dictionary.com
in reference to a throw, etc., "performed or done with the knuckles turned under," 1807, from under + hand (n.). Compare underhand. As "in secret," from 1825; as "with too few people," from 1834. Related: Underhandedly; underhandedness.
underlay (v.) Look up underlay at Dictionary.com
Old English under lecgan "to support by placing something beneath;" see under + lay (v.). Related: Underlaid; underlaying. Compare similarly formed Old High German Related: untarleccan, German unterlegen.
underlie (v.) Look up underlie at Dictionary.com
Old English under licgan "to be subordinate to, to submit to;" see under + lie (v.2). Meaning "to lie under or beneath" is attested from c.1600; figurative sense of "to be the basis of" is attested from 1852 (implied in underlying). Similar formation in Old High German untarliggan; German unterliegen.
underline (v.) Look up underline at Dictionary.com
1721, "to mark underneath or below with a line," from under + line (v.). Similar formation in Dutch onderlijnen. Related: Underlined; underlining. The noun is attested from 1888.
underling (n.) Look up underling at Dictionary.com
late Old English, "one who owes allegiance to a sovereign or ruler," from under + diminutive suffix -ling.
underlying (adj.) Look up underlying at Dictionary.com
1610s, present participle adjective from underlie.
undermine (v.) Look up undermine at Dictionary.com
c.1300, undermyne, "render unstable by digging at the foundation," from under + mine (v.). The figurative sense "injure by invisible, secret, or dishonorable means" is attested from early 15c. Similar formation in Dutch ondermijnen, Danish underminere, German unterminiren. The Old English verb was underdelfan. Related: Undermined; undermining.
undern (n.) Look up undern at Dictionary.com
an obsolete Old English and Middle English word for "morning;" in Old English originally "third hour of the day; 9 a.m." (corresponding to tierce). Hence underngeweorc, undernmete "breakfast." Common Germanic: Old Frisian unden, Old Saxon undorn, Middle Dutch onderen, Old High German untarn, Old Norse undorn; of uncertain origin. By extension, "period from 9 a.m. to noon;" but from 13c. shifting to "midday, noon" (as in undern-mete "lunch," 14c.); and by late 15c. to "late afternoon or early evening."
underneath (adv.) Look up underneath at Dictionary.com
Old English underneoðan, from under + neoðan "below" (see beneath).
undernourished (adj.) Look up undernourished at Dictionary.com
also under-nourished, 1820, from under + past participle of nourish (v.).
underpants (n.) Look up underpants at Dictionary.com
1931, from under + pants. Drove out drawers, knickers in this sense.
underpass (n.) Look up underpass at Dictionary.com
1904, American English, from under + pass (n.).
underpay (v.) Look up underpay at Dictionary.com
1817, from under + pay (v.). Related: underpaid (1762); underpaying.