Online Etymology Dictionary

acerbic (adj.): 1865, originally, and usually, figurative: "sour, harsh, severe" (of speech, manners, etc.), from Latin acerbus "harsh to the taste, sharp, bitter, sour" (see acerbity) + -ic.
acerbity (n.): 1570s, from Middle French acerbité, from Latin acerbitatem (nominative acerbitas) "harshness, sharpness, bitterness," from acerbus "bitter to taste, sharp, sour, tart" (related to acer "sharp;" compare Latin superbus "haughty," from super "above"), from Proto-Italic *akro-po- "sharp," from PIE *ak- "sharp" (see acrid). Earliest use in English is figurative, of "sharp and bitter" persons. Of tastes, from 1610s.
acetaminophen (n.): U.S. name for "para-acetylaminophenol," 1960, composed of syllables from the chemical name; in Britain, the same substance is paracetamol.
acetate (n.): 1827, "salt formed by combining acetic acid with a base," from Latin acetum "vinegar" (see acetic) + chemical suffix -ate (3). As a type of synthetic material, it is attested from 1920, short for acetate silk, etc.
acetic (adj.): 1808, from French acétique "pertaining to vinegar," from Latin acetum "vinegar" (properly vinum acetum "wine turned sour;" see vinegar), originally past participle of acere "be sour," related to acer "sharp" (see acrid).
aceto-: before vowels acet-, word-forming element from comb. form of acetic and generally indicating compounds from or related to acetic acid.
acetone (n.): colorless volatile liquid, 1839, literally "a derivative of acetic acid," from Latin acetum "vinegar" (see acetic) + Greek-based chemical suffix -one, which owes its use in chemistry to this word.
acetylene (n.): gaseous hydrocarbon, 1864, from French acétylène, coined by French chemist Marcelin-Pierre-Eugène Berthelot (1823-1907) from chemical ending -ene + acetyl, which was coined from acetic in 1839 by German chemist Justus von Liebig; see acetic. Liebig's coinage was in reference to a different radical; acetyl was transferred to its current sense in 1850s, but Berthelot's coinage was based on the original use of acetyl.
The name acetylene is an unfortunate one as the hydrocarbon is not directly related to the modern acetyl radical and the molecule ... contains a triple bond, not a double bond which the suffix -ene (q.v.) implies. [Flood, "Origins of Chemical Names," 1963]
ach (interj.): aspirated form of ah; in English often used to represent German or Celtic speech.
Achates: armor-bearer and faithful friend of Aeneas in the "Aeneid," hence sometimes used figuratively for "faithful friend." The name is from Greek akhates "agate" (see agate).
ache (v.): Old English acan "to ache, suffer pain," from Proto-Germanic *akanan, perhaps from a PIE root *ag-es- "fault, guilt," represented also in Sanskrit and Greek, perhaps imitative of groaning. The verb was pronounced "ake," the noun "ache" (as in speak/speech) but while the noun changed pronunciation to conform to the verb, the spelling of both was changed to ache c. 1700 on a false assumption of a Greek origin (specifically Greek akhos "pain, distress," which is rather a distant relation of awe (n.)). Related: Ached; aching.
ache (n.): early 15c., æche, from Old English æce, from Proto-Germanic *akiz, from same source as ache (v.).
Acheron: 1580s, fabled river of the Lower World in Greek mythology. The name perhaps means "marsh-like" (compare Greek akherousai "marshlike water"); the derivation from Greek akhos "woe" is considered folk etymology.
achieve (v.): early 14c., from Old French achever (12c.) "to finish, accomplish, complete," from phrase à chef (venir) "at an end, finished," or Vulgar Latin *accapare, from Late Latin ad caput (venire); both the French and Late Latin phrases meaning literally "to come to a head," from stem of Latin caput "head" (see capitulum).
The Lat. caput, towards the end of the Empire, and in Merov[ingian] times, took the sense of an end, whence the phrase ad caput venire, in the sense of to come to an end .... Venire ad caput naturally produced the Fr. phrase venir à chef = venir à bout. ... From this chief, O.Fr. form of chef (q.v.) in sense of term, end, comes the Fr. compd. achever = venir à chef, to end, finish. [Auguste Brachet, "An Etymological Dictionary of the French Language," transl. G.W. Kitchin, Oxford, 1878]
Related: Achieved; achieving.
achievement (n.): late 15c., "act of completing" (something), from Middle French achèvement "a finishing," noun of action from Old French achever "to finish, accomplish" (see achieve). Meaning "thing achieved" is recorded from 1590s.
Achilles: Greek hero of the Trojan War stories, son of Thetis and Peleus, his name is perhaps a compound of akhos "pain, grief" (see awe) + laos "the people, a people" (see lay (adj.)); or else it is from a pre-Greek language.
Achilles tendon (n.): from Modern Latin tendo Achillis, first used by German surgeon Lorenz Heister (1683-1758) and so-called in reference to the one vulnerable spot of the great Greek hero Achilles, whose mother held him by the heel when she dipped him in the River Styx to render him invulnerable (though this story is not in Homer and not found before 1c. C.E.). Earlier Achilles' sinew, from Modern Latin chorda Achillis, coined 1693 by Dutch anatomist Philip Verheyden when dissecting his own amputated leg. Hence figurative use of heel of Achillies for "vulnerable spot" (1810).
achromatic (adj.): 1766, from Latinized form of Greek akhromatos "colorless," from a-, privative prefix (see a- (3)), + khromat-, comb. form of khroma "color" (see chroma) + -ic.
achtung: German word used to command attention, from German achtung, from acht (n.) "attention, care, heed, consideration," achten (v.) "pay attention to, regard, esteem, respect," from Old High German ahton "pay attention to," a general Germanic word akin to Old English eahtian "to estimate, esteem, consider, praise," but with no living native descendants in English.
achy (adj.): 1875, first recorded in George Eliot's letters, from ache (n.) + -y (2). Middle English had akeful "painful" (early 15c.). Related: Achily; achiness.
acid (adj.): 1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp," adjective of state from acere "to be sour," from PIE root *ak- "sharp, pointed" (see acrid). Figurative use from 1775; applied to intense colors from 1916. Acid test is American English, 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in reference to England.
acid (n.): 1690s, from acid (adj.). Slang meaning "LSD-25" first recorded 1966 (see LSD).
When I was on acid I would see things that looked like beams of light, and I would hear things that sounded an awful lot like car horns. [Mitch Hedberg, 1968-2005, U.S. stand-up comic]
Acid rock (type played by or listen to by people using LSD) is also from 1966; acid house dance music style is 1988, probably from acid in the hallucinogenic sense + house "dance club DJ music style."
acidic (adj.): 1877, originally in geology; see acid (n.) + -ic.
acidity (n.): 1610s, from French acidité (26c.) or directly from Latin aciditatem (nominative aciditas), noun of quality from Latin acidus (see acid (adj.)).
acidophilus (adj.): 1920, used of milk fermented by acidophilic bacteria, from acidophil (1900), a hybrid word, from Latin acidus "acidic" (see acid (adj.)) + Greek philos "loving" (see -phile); the bacteria so called because they stain easily with an acid dye.
acidulous (adj.): 1769, "sub-acidic," used figuratively for "sour-tempered;" from Latin acidulus "slightly sour," a diminutive of acidus (see acid (adj.)).
ack: British oral code for letter A in wireless and telephone communication, 1898; hence ack-ack "anti-aircraft" (gun, fire, etc.).
ack-ack: 1939, representing A.A., the military abbreviation for anti-aircraft (see ack).
acknowledge (v.): 1550s, a blend of Middle English aknow (from Old English oncnawan "understand," from on + cnawan "recognize;" see know) and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). In the merger, a parasitic -c- slipped in, so that while the kn- became a simple "n" sound (as in know), the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.
acknowledgement (n.): 1590s, "act of acknowledging," from acknowledge + -ment. "An early instance of -ment added to an orig. Eng. vb." [OED]. Meaning "token of due recognition" is recorded from 1610s.
acknowledgment (n.): alternative form of acknowledgement. OED deems it "a spelling more in accordance with Eng. values of letters."
acme (n.): "highest point," 1560s, from Greek akme "(highest) point, edge; peak of anything," from PIE root *ak- "sharp" (see acrid). Written in Greek letters until c. 1620. The U.S. grocery store chain was founded 1891 in Philadelphia, Pennsylvania.
acne (n.): 1813, from Modern Latin, from aknas, a 6c. Latin clerical misreading of Greek akmas, accusative plural of akme "point" (see acme). The "pointed" pimples are the source of the medical use.
acolyte (n.): early 14c., "inferior officer in the church," from Old French acolite or directly from Medieval Latin acolytus (Late Latin acoluthos), from Greek akolouthos "following, attending on," literally "having one way," from a- "together with," copulative prefix, + keleuthose "a way, road, path, track," from PIE *qeleu- (source also of Lithuanian kelias "way"). In late Old English as a Latin word.
acomia (n.): "baldness," Modern Latin, from Greek akomos "hairless," from a-, privative prefix (see a- (3)) + kome "hair" (see comet).
aconite (n.): poisonous plant (also known as monkshood and wolf's bane), 1570s, from French aconit, from Latin aconitum, from Greek akoniton, which is of unknown origin.
acorn (n.): Old English æcern "nut," common Germanic (cognates: Old Norse akarn, Dutch aker, Low German ecker "acorn," German Ecker, Gothic akran "fruit"), originally the mast of any forest tree, and ultimately related (via notion of "fruit of the open or unenclosed land") to Old English æcer "open land," Gothic akrs "field," Old French aigrun "fruits and vegetables" (from Frankish or some other Germanic source); see acre.

The sense gradually restricted in Low German, Scandinavian, and English to the most important of the forest produce for feeding swine, the mast of the oak tree. Spelling changed 15c.-16c. by folk etymology association with oak (Old English ac) and corn (n.1).
acoustic (adj.): c. 1600, from French acoustique, from Greek akoustikos "pertaining to hearing," from akoustos "heard, audible," verbal adjective from akouein "to hear," probably from copulative prefix a- (see a- (3)) + koein "to mark, perceive, hear," from PIE *kous- "to hear," which is perhaps from root *(s)keu- "to notice, observe" (see caveat). Acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.
acoustics (n.): 1680s, "science of sound," from acoustic (also see -ics). Meaning "acoustic properties" of a building, etc., attested from 1885.
acquaint (v.): early 13c., from Old French acointier "make known, make acquaintance of," from Vulgar Latin accognitare "to make known," from Latin accognitus "acquainted with," past participle of accognoscere "know well," from ad- "to" (see ad-) + cognitus, past participle of cogniscere "come to know," from com- "with" (see com-) + gnoscere "know" (see notice (n.)). Originally reflexive, "to make oneself known;" sense of "to gain for oneself personal knowledge of" is from early 14c. Related: Acquainted; acquainting.
acquaintance (n.): c. 1300, "state of being acquainted;" late 14c., "person with whom one is acquainted;" also "personal knowledge;" from Old French acointance "acquaintance, friendship, familiarity," noun of action from acointer (see acquaint). Acquaintant (17c.), would have been better in the "person known" sense but is now obsolete.
acquainted (adj.): early 13c., "personally known;" past participle adjective from acquaint (v.). Of skills, situations, etc., from late 15c.
I have been one acquainted with the night.
I have walked out in rain — and back in rain.
I have outwalked the furthest city light.

[Robert Frost, from "Acquainted with the Night"]
acquiesce (v.): 1610s, from Middle French acquiescer (16c.), from Latin acquiescere "to become quiet, remain at rest," thus "be satisfied with," from ad- "to" (see ad-) + quiescere "to become quiet," from quies (genitive quietis) "rest, quiet" (see quiet (n.)). Related: Acquiesced; acquiescing.
acquiescence (n.): 1630s, "act of acquiescing," from French acquiescence, noun of action from acquiescer (see acquiesce). Meaning "silent consent" is recorded from 1640s.
acquiescent (adj.): 1690s (implied in acquiescently), from Latin acquiescentem (nominative acquiescens), present participle of acquiescere (see acquiesce).
acquire (v.): mid-15c., acqueren, from Old French aquerre "acquire, gain, earn, procure," from Vulgar Latin *acquaerere, corresponding to Latin acquirere "to seek in addition to" (see acquisition). Reborrowed in current form from Latin c. 1600. Related: Acquired; acquiring.
acquired (adj.): c. 1600, "gained by effort," past participle adjective from acquire. Of diseases, "occurring after birth, thus not dependent on heredity," 1842 (opposed to congenital). Acquired taste is attested from 1734.
acquisition (n.): late 14c., "act of obtaining," from Old French acquisicion (13c.) or directly from Latin acquisitionem (nominative acquisitio), noun of action from past participle stem of acquirere "get in addition, accumulate," from ad- "extra" (see ad-) + quaerere "to seek to obtain" (see query (v.)). Meaning "thing obtained" is from late 15c. The vowel change of -ae- to -i- in Latin is due to a Latin phonetic rule involving unaccented syllables in compounds.
acquisitive (adj.): 1630s, "owned through acquisition," from Latin acquisit-, past participle stem of acquirere (see acquisition) + -ive. Meaning "given to acquisition, avaricious" is from 1826 (implied in acquisitiveness). Related: Acquisitively (1590s).
acquit (v.): early 13c., "to satisfy a debt" (either for oneself or on behalf of another), from Old French aquiter "pay, pay up, settle a claim" (12c.), from a "to" (see ad-) + quite "free, clear" (see quit (adj.)). Meanings "set free from charges" and "to discharge one's duty" both recorded from late 14c. Related: Acquitted; acquitting.