Online Etymology Dictionary

aplasia (n.): 1885, medical Latin, from Greek a-, privative prefix (see a- (3)), + -plasia.
aplenty (adj.): 1830, originally U.S., from a- (1) + plenty (n.). First attested in writings of J. Fenimore Cooper.
aplomb (n.): "assurance, confidence," 1828, from French aplomb (16c.), literally "perpendicularity," from phrase à plomb "poised upright, balanced," literally "on the plumb line," from Latin plumbum "(the metal) lead" (see plumb (n.)), of which the weight at the end of the line was made.
apnea (n.): "suspension of breathing," also apnoea, 1719, Modern Latin, from Greek apnoia "absence of respiration," from apnos "without breathing," from a- "not" (see a- (3)) + pnein "to breathe" (see pneuma).
apneic (adj.): 1883, from apnea + -ic.
apo koinu: Greek, literally "in common." Applied to sentences with one subject and two predicates; a formation rare in modern English, though it occurs more often in Old English. Cf. koine.
apo-: before vowels ap-, word-forming element meaning "from, away from, separate, free from," from Greek apo "from, away from; after; in descent from," in compounds, "from, asunder, away, off; finishing, completing; ceasing from; back again," from PIE root *apo- "off, away" (cf. Sanskrit apa "away from," Avestan apa "away from," Latin ab "away from, from," Gothic af, Old English of "away from").
apocalypse (n.): late 14c., "revelation, disclosure," from Church Latin apocalypsis "revelation," from Greek apokalyptein "uncover, disclose, reveal," from apo- "from" (see apo-) + kalyptein "to cover, conceal" (see Calypso). The Christian end-of-the-world story is part of the revelation in John of Patmos' book "Apokalypsis" (a title rendered into English as "Apocalypse" c.1230 and "Revelations" by Wyclif c.1380).

Its general sense in Middle English was "insight, vision; hallucination;" meaning "a cataclysmic event" is modern. As agent nouns, apocalypst (1829), apocalypt (1834), and apocalyptist (1835) have been tried.
apocalyptic (adj.): 1660s, "pertaining to the 'Revelation of St. John' in the New Testament," from Greek apokalyptikos, from apokalyptein (see apocalypse). Meaning "pertaining to the imminent end of the world" evolved by 1880s.
Apocrypha: late 14c., neuter plural of Late Latin apocryphus "secret, not approved for public reading," from Greek apokryphos "hidden; obscure," thus "(books) of unknown authorship" (especially those included in the Septuagint and Vulgate but not originally written in Hebrew and not counted as genuine by the Jews), from apo- "away" (see apo-) + kryptein "to hide" (see crypt). Properly plural (the single would be Apocryphon or apocryphum), but commonly treated as a collective singular.
apocryphal (adj.): 1580s, "of doubtful authenticity," from Apocrypha + -al (1). Middle English had apocrive (late 14c.) in same sense.
apodal (adj.): 1769, from Greek apous (from apod- "footless," from a-, privative prefix, see a- (3), + pous "foot;" see foot (n.)) + -al.
apodictic (adj.): "clearly demonstrated," 1650s, from Latin apodicticus, from Greek apodeiktikos, from apodeiktos, verbal adjective of apodeiknynai "to show off, demonstrate," literally "to point away from" (other objects, at one), from apo "off, away" (see apo-) + deiknynai "to show" (see diction).
apodyterium (n.): 1690s, from Latin apodyterium "undressing room" (in a bath house), from Greek apodyterion "undressing room," from apodyein "to put off, undress," from apo- "off" (see apo-) + dyein "to put on, enter, go in."
apogee (n.): "point at which the moon is farthest from the earth," 1590s, from French apogée, from Latin apogaeum, from Greek apogaion, neuter adjective, "away from the earth," a term from Ptolemaic astronomy, from apo "off, away" (see apo-) + gaia/ge "earth" (see Gaia). Adjective forms are apogeal, apogean.
apolitical (adj.): 1947, from a- "not" (see a- (3)) + political.
Apollo: Olympian deity, god of music, poetry, medicine, etc., later identified with Helios, the sun god; the name is a Latin form of Greek Apollon, said to be perhaps related to an obsolete Greek verb meaning "to drive away" (evil, etc.) [Klein, citing Usener].
Apollonian (adj.): 1660s, from Apollo (Greek Apollon) + -ian. The Greek adjective was Apollonios. The word also is attested in English as Apollinarian (1753), Apolline (c.1600).
Apollyon: destroying angel of the bottomless pit (a name sometimes given to the Devil), late 14c., from present participle of Greek apollyein "to destroy utterly" (from apo- "from, away from" + olluein "to destroy"); a translation of Hebrew Abaddon (q.v.).
apologetic (adj.): 1640s, "vindicatory," from French apologétique, from Latin apologeticus, from Greek apologetikos "defensible," from apologeisthai (see apology). Meaning "regretfully acknowledging failure" is from 1855. As a noun, "formal defense," from early 15c. Related: Apologetics (c.1753).
apologia (n.): see apology.
apological (adj.): c.1600; see apology + -ical.
apologise (v.): chiefly British English spelling of apologize; for suffix, see -ize. Related: Apologised; apologising.
apologist (n.): 1630s, from French apologiste, from apologie (see apology).
apologize (v.): 1590s, "to speak in defense of;" see apology + -ize. Main modern sense "to regretfully acknowledge" is attested by 1725. The Greek equivalent, apologizesthai, meant simply "to give an account." Related: Apologized; apologizing.
apologue (n.): "moral fable," 1550s, from French apologue, from Latin apologus, from Greek apologos, from apo- "off, away from" (see apo-) + logos "speech" (see lecture (n.)). Literally, "(that which comes) from a speech."
apology (n.): early 15c., "defense, justification," from Late Latin apologia, from Greek apologia "a speech in defense," from apologeisthai "to speak in one's defense," from apologos "an account, story," from apo- "from, off" (see apo-) + logos "speech" (see lecture (n.)).

The original English sense of "self-justification" yielded a meaning "frank expression of regret for wrong done," first recorded 1590s, but this was not the main sense until 18c. The old sense tends to emerge in Latin form apologia (first attested in English 1784), especially since J.H. Newman's "Apologia pro Vita Sua" (1864).
aponeurosis (n.): 1670s, from Latin, from Greek aponeurosis, from aponeuroein, from apo- "off, away from" (see apo-) + neuron "sinew" (see neuro-).
apophatic (adj.): "involving a mention of something one feigns to deny; involving knowledge obtained by negation," 1850, from Greek apophatikos, from apophasis "denial, answer," from apophanai "to speak off," from apo- "off" (see apo-) + phanai "to speak," related to pheme "voice" (see fame (n.)).
apophthegm (n.): see apothegm.
apoplectic (adj.): 1610s, "involving apoplexy," from French apoplectique (16c.), from Latin apoplecticus, from Greek apoplektikos "disabled by a stroke, crippled, struck dumb," from apoplektos, verbal adjective of apoplessein (see apoplexy). Meaning "showing symptoms of apoplexy" (1721) gradually shaded into "enraged, very angry."
apoplexy (n.): late 14c., "sudden fit of paralysis and dizziness," from Old French apoplexie or directly from Late Latin apoplexia, from Greek apoplexia, from apoplessein "to strike down and incapacitate," from apo- "off" (see apo-), in this case probably an intensive prefix, + plessein "hit" (cf. plague (n.), also with a root sense of "stricken"). The Latin translation, sideratio, means "disease caused by a constellation."
aporetic (adj.): c.1600, from French aporetique, from Greek aporetikos, from aporeein "to be at a loss," from aporos "impassable, impracticable, very difficult; hard to deal with; at a loss," from a-, privative prefix (see a- (3)), + poros "passage" (see pore (n.)).
aporia (n.): 1580s, from Latin, from Greek aporia, noun of state from aporos (see aporetic).
aposiopesis (n.): rhetorical artifice wherein the speaker suddenly breaks off in the middle of a sentence, 1570s, from Latin, from Greek aposiopesis "a becoming silent," also as a rhetorical figure, from apo- (see apo-) + siope "silence."
apostasy (n.): late 14c., "renunciation, abandonment or neglect of established religion," from Latin apostasia, from later Greek apostasia, from apostasis "revolt, defection," literally "a standing off" (see apostate). General (non-religious) sense is attested from 1570s.
apostate (n.): mid-14c., "one who forsakes his religion or faith," from Old French apostate (Modern French apostat) and directly from Late Latin apostata, from Greek apostasia "defection, desertion, rebellion," from apostenai "to defect," literally "to stand off," from apo- "away from" (see apo-) + stenai "to stand." Used in non-religious situations (politics, etc.) from mid-14c.
apostate (adj.): late 14c.; see apostate (n.).
apostatize (v.): 1610s, from Late Latin apostatizare, earlier apostatare, from apostata (see apostate). Related: Apostatized; apostatizing. The past participle form apostazied is attested from late 14c.
apostille (n.): "note, especially on text of the Bible," also apostil, 1520s, from French apostille (15c.), probably from Medieval Latin postilla, which probably represents Latin post illa, literally "after those."
apostle (n.): Old English apostol "messenger," especially the 12 witnesses sent forth by Jesus to preach his Gospel, from Late Latin apostolus, from Greek apostolos "messenger, person sent forth," from apostellein "send away, send forth," from apo- "from" (see apo-) + stellein in its secondary sense of "to send," from PIE *stel-yo-, suffixed form of root *stel- "to put, stand," with derivatives referring to a standing object or place (see stall (n.1)). Cf. epistle.

The current form of the word, predominant since 16c., is influenced by Old French apostle (12c.), from the same Late Latin source. Figurative sense of "chief advocate of a new principle or system" is from 1810. Apostles, short for "The Acts and Epistles of the Apostles," is attested from c.1400.
apostolic (adj.): early 15c., from French apostolique or directly from Church Latin apostolicus, from Greek apostolikos, from apostolos (see apostle). Apostolical also is early 15c.
apostrophe (n.): mark indicating omitted letter, 1580s, from Middle French apostrophe, from Late Latin apostrophus, from Greek apostrophos (prosoidia) "(the accent of) turning away," thus, a mark showing where a letter has been omitted, from apostrephein "avert, turn away," from apo- "from" (see apo-) + strephein "to turn" (see strophe).

In English, the mark often represents loss of -e- in -es, possessive ending. It was being extended to all possessives, whether they ever had an -e- or not, by 18c. Greek also used this word for a "turning aside" of an orator in speech to address some individual, a sense first recorded in English 1530s.
apothecary (n.): mid-14c., "shopkeeper, especially one who stores, compounds, and sells medicaments," from Old French apotecaire (13c., Modern French apothicaire), from Late Latin apothecarius "storekeeper," from Latin apotheca "storehouse," from Greek apotheke "barn, storehouse," literally "a place where things are put away," from apo- "away" (see apo-) + tithenai "to put," from PIE root *dhe- "to put, to do" (see factitious). Same root produced French boutique and Spanish bodega. Cognate compounds produced Sanskrit apadha- "concealment," Old Persian apadana- "palace."

Drugs and herbs being among the chief items of non-perishable goods, the meaning narrowed 17c. to "druggist" (Apothecaries' Company of London separated from the Grocers' in 1617). Apothecaries formerly were notorious for "the assumed gravity and affectation of knowledge generally put on by the gentlemen of this profession, who are commonly as superficial in their learning as they are pedantic in their language" [Francis Grose, "A Classical Dictionary of the Vulgar Tongue," 1796]. Hence, Apothecary's Latin, barbarously mangled, also known as Dog Latin.
apothegm (n.): "pithy saying," 1550s, from Greek apophthegma "terse, pointed saying," literally "something clearly spoken," from apophthengesthai "to speak one's opinion plainly," from apo- "from" (see apo-) + phthengesthai "to utter." See aphorism for nuances of usage. Spelling apophthegm, restored by Johnson, is preferred in England, according to OED.
apotheosis (n.): 1600s, from Late Latin apotheosis "deification," from Greek apotheosis, from apotheoun "deify, make (someone) a god," from apo- special use of this prefix, meaning, here, "change" + theos "god" (see Thea).
apotheosize (v.): 1760; see apotheosis + -ize. Related: Apotheosized; apotheosizing. Earlier in same sense was apotheose (1670s).
apothesis (n.): 1811, from Greek apothesis "a laying up in store; a putting aside," noun of action from apotithenai "to lay aside," from apo- "off, away" (see apo-) + tithenai (see theme).
apotropaic (adj.): 1883, with -ic + Greek apotropaios "averting evil," from apotrepein "to turn away, avert," from apo- "off, away" (see apo-) + trepein "to turn" (see trope).
app (n.): computerese shorthand for application, attested by 1992.