Online Etymology Dictionary

acquittal (n.): early 15c., "payment of debt or retribution;" see acquit + -al (2). Sense of "a release from debt or obligation" is from mid-15c.; that of "freeing from charge or offense" (by legal process) is from 1530s.
acquitted (adj.): "freed, exonerated," 1670s, past participle adjective from acquit (v.). Formerly in this sense was acquit (late 14c.), perhaps on analogy of past participles such as hit.
acre (n.): Old English æcer "tilled field, open land," from Proto-Germanic *akraz "field, pasture" (source also of Old Norse akr, Old Saxon akkar, Old Frisian ekker, Middle Dutch acker, Dutch akker, Old High German achar, German acker, Gothic akrs), from PIE *agro- "field" (source also of Latin ager "field, land," Greek agros, Sanskrit ajras "plain, open country").

Originally in English without reference to dimension; in late Old English the amount of land a yoke of oxen could plow in a day, afterward defined by statute as a piece 40 poles by 4, or an equivalent shape (5 Edw. I, 31 Edw. III, 24 Hen. VIII). Original sense retained in God's acre "churchyard."
acreage (n.): 1859, from acre + -age.
acrid (adj.): 1712, formed irregularly from Latin acer (fem. acris) "sharp, pungent, bitter, eager, fierce," from PIE *akri- "sharp," from root *ak- "be sharp, rise (out) to a point, pierce" (source also of Oscan akrid (ablative singular) "sharply;" Greek akis "sharp point," akros "at the farthest point, highest, outermost," akantha "thorn," akme "summit, edge;" also oxys "sharp, bitter;" Sanskrit acri- "corner, edge," acani- "point of an arrow," asrih "edge;" Lithuanian ašmuo "sharpness," akstis "sharp stick;" Old Lithuanian aštras, Lithuanian aštrus "sharp;" Old Church Slavonic ostru, Russian óstryj "sharp;" Old Irish er "high;" Welsh ochr "edge, corner, border;" Old Norse eggja "goad;" Old English ecg "sword"). The -id suffix probably is in imitation of acid. Acrious (1670s) is a correct formation, but seldom seen.
acrimonious (adj.): 1610s, "acrid," from French acrimonieux, from Medieval Latin acrimoniosus, from Latin acrimonia "sharpness" (see acrimony). Of dispositions, debates, etc., from 1775. Related: Acrimoniously; acrimoniousness.
acrimony (n.): 1540s, "quality of being acrid," from Middle French acrimonie or directly from Latin acrimonia "sharpness, pungency of taste," figuratively "acrimony, severity, energy," from acer "sharp" (fem. acris, neuter acre; see acrid) + -monia suffix of action, state, condition. Figurative extension to "sharpness of temper" is first recorded 1610s.
acro-: word-forming element meaning "highest, topmost, at the extremities," before vowels, acr-, from Greek akro- "pertaining to an end, extreme," comb. form of akros "at the end, at the top" (see acrid).
acrobat (n.): 1825, from French acrobate (14c.), "tightrope-walker," and directly from Greek akrobates "rope dancer, gymnastic performer," which is related to akrobatos "going on tip-toe, climbing up high," from akros "topmost, at the point end" (see acrid) + stem of bainein "walk, go" (see come).
acrobatic (adj.): 1848; see acrobat + -ic. Related: Acrobatically.
acrobatics (n.): 1859, from acrobatic; also see -ics. Also acrobatism (1864). In early 20c. acrobacy (from French acrobacie) sometimes was used.
acromegaly (n.): "gigantism due to activity of pituitary after normal growth has ceased," 1886, from French acromégalie, from medical Latin acromegalia, from Greek akron "extremity" (see acrid) + megas "great" (fem. megale; see mickle). Said in contemporary literature to have been coined 1885 by French physician Dr. Pierre Marie.
acronym (n.): word formed from the first letters of a series of words, 1943, American English coinage from acro- + -onym "name" (abstracted from homonym; see name (n.)). But for cabalistic esoterica and acrostic poetry, the practice was practically non-existent before 20c. For distinction of usage (not maintained on this site), see initialism.
acrophobe (n.): "one suffering from acrophobia," 1895, from acrophobia; also see -phobe.
acrophobia (n.): "morbid fear of heights," 1887, medical Latin, from Greek akros "at the end, the top" (see acrid) + -phobia "fear." Coined by Italian physician Dr. Andrea Verga in a paper describing the condition, from which Verga himself suffered.
In this paper, read somewhat over a year ago at the congress of alienists at Pavia, the author makes confession of his own extreme dread of high places. Though fearless of the contagion of cholera, he has palpitations on mounting a step-ladder, finds it unpleasant to ride on the top of a coach or to look out of even a first-story window, and has never used an elevator. ["American Journal of Psychology," Nov. 1888, abstract of Verga's report]
acropolis (n.): 1660s, from Latinized form of Greek akropolis "citadel" (especially that of Athens), from akros "highest, upper" (see acrid) + polis in its older sense of "citadel; enclosed space, often on high ground" (see polis).
across (adv.): early 14c., acros, earlier a-croiz (c. 1300), from Anglo-French an cros "in a crossed position," literally "on cross" (see cross (n.)). Prepositional meaning "from one side to another" is first recorded 1590s; meaning "on the other side (as a result of crossing)" is from 1750. Phrase across the board originally is from horse-racing, in reference to a bet of the same amount of money on a horse to win, place, or show.
acrostic (n.): short poem in which the initial letters of the lines, taken in order, spell a word or phrase, 1580s, from Medieval Latin acrostichis, from Greek akrostikhis, from akros "at the end, outermost" (see acrid) + stikhos "line of verse," literally "row" (see stair).
acrylic (adj.): 1855, "of or containing acryl," name of a radical from acrolein (1843), the name of a liquid in onions and garlic that makes eyes tear, from Latin acer "sharp" (see acrid) + olere "to smell" (see odor) + -in (see -ine (2)). With adjectival suffix + -ic. Modern senses often short for acrylic fiber, acrylic resin, etc.
act (n.): late 14c., "a thing done," from Old French acte "(official) document," and directly from Latin actus "a doing, a driving, impulse; a part in a play, act," and actum "a thing done," originally a legal term, both from agere "to do, set in motion, drive, urge, chase, stir up," from PIE root *ag- "to drive, draw out or forth, move" (source also of Greek agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agogos "leader;" Sanskrit ajati "drives," ajirah "moving, active;" Old Norse aka "to drive;" Middle Irish ag "battle").

Theatrical ("part of a play," 1510s) and legislative (early 15c.) senses of the word also were in Latin. Meaning "display of exaggerated behavior" is from 1928. In the act "in the process" is from 1590s, perhaps originally from the 16c. sense of the act as "sexual intercourse." Act of God "uncontrollable natural force" recorded by 1726.
An act of God is an accident which arises from a cause which operates without interference or aid from man (1 Pars. on Cont. 635); the loss arising wherefrom cannot be guarded against by the ordinary exertions of human skill and prudence so as to prevent its effect. [William Wait, "General Principles of the Law," Albany, 1879]
act (v.): mid-15c., "to act upon or adjudicate" a legal case; 1590s in the theatrical sense, from Latin actus, past participle of agere (see act (n.)). To act up "be unruly" is from 1903. To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires." Related: Acted; acting.
Actaeon: in Greek mythology, the name of the hunter who discovered Artemis bathing and was changed by her to a stag and torn to death by his hounds. The name is of unknown origin. Sometimes used figuratively in 17c. for "a cuckold" (because of his "horns").
acting (adj.): 1590s, "putting forth activity," present participle adjective from act (v.). Meaning "performing temporary duties" is from 1797.
acting (n.): c. 1600, "performance of deeds;" 1660s, "performance of plays;" verbal noun from present participle of act (v.). Acting out in psychology is from 1945.
actinium (n.): radioactive element discovered in 1899, from Greek actin-, comb. form of aktis (genitive aktinos) "ray, radiance" (see actino-) + chemical suffix -ium.
actino-: before vowels actin-, word-forming element meaning "pertaining to rays," from Greek aktis (genitive aktinos) "ray, radiance;" perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early."
action (n.): mid-14c., "cause or grounds for a lawsuit," from Anglo-French accioun, Old French accion (12c.) "action, lawsuit, case," from Latin actionem (nominative actio) "a putting in motion; a performing, doing," noun of action from past participle stem of agere "to do" (see act (n.)). Sense of "something done, an act, deed" is late 14c. Meaning "fighting" is from c. 1600. As a film director's command, it is attested from 1923. Meaning "excitement" is recorded from 1968. Phrase actions speak louder than words is attested from 1731.
actionable (adj.): 1590s; from action + -able.
activate (v.): 1620s; see active + -ate (2). Related: Activated; activating.
activation (n.): 1906, noun of action from activate (v.).
active (adj.): mid-14c., "given to worldly activity" (opposed to contemplative or monastic), from Old French actif (12c.) or directly from Latin activus, from actus (see act (n.)). As "capable of acting" (opposed to passive), from late 14c. Meaning "energetic, lively" is from 1590s; that of "working, effective, in operation" is from 1640s. Active voice is recorded from 1765 (grammatical use of active dates from mid-15c.).
actively (adv.): c. 1400, "secularly," from active + -ly (2). Meaning "vigorously" is early 15c.
activism (n.): 1920 in the political sense; see activist + -ism. Earlier (1907) it was used in reference to a philosophical theory.
activist (n.): "one who advocates a doctrine of direct action," 1915; from active + -ist. Originally in reference to political forces in Sweden advocating abandonment of neutrality in World War I and active support for the Central Powers.
activities (n.): in schoolwork sense, 1923, American English, from activity.
activity (n.): c. 1400, "active or secular life," from Old French activité, from Medieval Latin activitatem (nominative activitas), a word in Scholastic philosophy, from Latin activus (see active). Meaning "state of being active, briskness, liveliness" recorded from 1520s; that of "capacity for acting on matter" is from 1540s.
actor (n.): late 14c., "an overseer, guardian, steward," from Latin actor "an agent or doer," also "theatrical player," from past participle stem of agere (see act (n.)). Mid-15c. as "a doer, maker," also "a plaintiff." Sense of "one who performs in plays" is 1580s, originally applied to both men and women.
actress (n.): 1580s, "female who does something;" see actor + -ess; stage sense is from 1700. Sometimes French actrice was used.
Acts: short for "Acts of the Apostles" in New Testament, from 1530s.
actual (adj.): early 14c., "pertaining to an action," from Old French actuel "now existing, up to date" (13c.), from Late Latin actualis "active, pertaining to action," adjectival form of Latin actus (see act (n.)). The broader sense of "real, existing" (as opposed to potential, ideal, etc.) is from late 14c.
actuality (n.): late 14c., "power, efficacy," from Old French actualite and directly from Medieval Latin actualitatem (nominative actualitas), from Late Latin actualis (see actual). A Latin loan-translation of Greek energeia. Meaning "state of being real" is from 1670s (actualities "existing conditions" is from 1660s).
Mod. use of actuality in the sense of realism, contact with the contemporary, is due to Fr. actualité, from actuel, which does not mean actual, real, but now existing, up to date. [Weekley]
actualization (n.): 1824, noun of action from actualize.
actualize (v.): 1810, first attested in Coleridge, from actual + -ize. Related: Actualized; actualizing.
actually (adv.): early 15c., "in fact, in reality" (as opposed to in possibility), from actual + -ly (2). Meaning "actively, vigorously" is from mid-15c.; that of "at this time, at present" is from 1660s. As an intensive added to a statement and suggesting "as a matter of fact, really, in truth" it is attested from 1762.
actuarial (adj.): 1853, from actuary + -al (1). Related: Actuarially.
actuary (n.): 1550s, "registrar, clerk," from Latin actuarius "copyist, account-keeper," from actus "public business" (see act (n.)). Modern insurance office meaning first recorded 1849.
actuate (v.): 1590s, from Medieval Latin actuatus, past participle of actuare, from Latin actus (see act (n.)). Related: Actuated; actuating.
acuity (n.): early 15c., from Middle French acuité (16c.) or directly from Medieval Latin acuitatem (nominative acuitas) "sharpness," from Latin acuere "to sharpen," related to acus "needle," acuere "to sharpen," from PIE root *ak- "rise to a point, be sharp" (see acrid).
acumen (n.): 1530s, from Latin acumen "a point, sting," hence "mental sharpness, shrewdness," from acuere "to sharpen" (see acuity).
acupressure (n.): 1859, from Latin acus "needle" (see acuity) + pressure (n.).