Online Etymology Dictionary

afterthought (n.): 1660s, from after + thought.
afterward (adv.): Old English æftanweard, from æftan "after" (see aft) + -weard suffix indicating direction (see -ward); nautical use as aftward, then expanded by influence of after; variant afterwards shows adverbial genitive.
afterwards (adv.): c. 1300, from afterward (q.v.) + adverbial genitive -s; originally a Northern form.
afterword (n.): 1890, a Saxonist subsitute for epilogue; from after + word (n.).
ag (n.): abbreviation of agriculture, attested from 1918, American English.
aga (n.): title of rank, especially in Turkey, c. 1600, from Turkish agha "chief, master, lord," related to East Turk. agha "elder brother."
again (adv.): late Old English agan, from earlier ongean "toward, opposite, against, in exchange for," from on "on" (see on) + -gegn "against, toward," compounded for a sense of "lined up facing, opposite," and "in the opposite direction, returning." For -gegn, compare Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to."

In Old English, eft was the main word for "again" (see eftsoons), but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. Differentiated from against 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted).
against (adv.): early 12c., agenes "in opposition to," a southern variant of agen "again" (see again), with adverbial genitive. The parasitic -t turned up mid-14c. and was standard by early 16c., perhaps from influence of superlatives.
Agamemnon: king of Mycenae, leader of the Greeks in the Trojan War, his name perhaps represents Greek Aga-medmon, literally "ruling mightily," from agan "very much" + medon "ruler" (see meditation).
agape (n.): c. 1600, from Greek agape "brotherly love, charity," from agapan "greet with affection, love," which is of unknown origin. Agape was used by early Christians for their "love feast" held in connection with the Lord's Supper. In modern use, often in simpler sense of "Christian love" (1856, frequently opposed to eros as "carnal or sensual love").
agape (adv.): 1660s, from a- (1) + gape (v.).
agate (n.): 1560s, from Middle French agathe (16c.), from Latin achates, from Greek akhates, the name of a river in Sicily where the stones were found (Pliny). But the river could as easily be named for the stone.

The earlier English form of the word, achate (early 13c.), was directly from Latin. Figurative sense of "a diminutive person" (c. 1600) is from the now-obsolete meaning "small figures cut in agates for seals," preserved in typographer's agate (1838), the U.S. name of the 5.5-point font called in Great Britain ruby. Meaning "toy marble made of glass resembling agate" is from 1843 (colloquially called an aggie).
Agatha: fem. proper name, from Latin, from Greek Agathe, fem. of agathos "good," which is of unknown origin. Never a popular name in U.S., and all but unused there since 1940.
agathism (n.): the doctrine that all things tend toward the good, 1830, from agathist + -ism.
agathist (n.): 1816, from Greek agathos "good" (see Agatha) + -ist.
Doctor Kearney, who formerly, with so much reputation, delivered lectures in this place on the history of Rome, observed to me once, that he was not an optimist, but an "agathist"; that he believed that every thing tended to good, but did not think himself competent to determine what was absolutely the best. The distinction is important, and seems to be fatal to the system of Optimism. [George Miller, "Lectures on the Philosophy of Modern History," Dublin, 1816]
agave (n.): American aloe plant, 1797, from Latin Agave, from Greek Agaue, proper name in mythology (mother of Pentheus), from agauos "noble," perhaps from agasthai "wonder at," from gaiein "to rejoice, exult," with intensive prefix a-. The name seems to have been taken generically by botanists, the plant perhaps so called for its "stately" flower stem.
age (n.): late 13c., "long but indefinite period in human history," from Old French aage (11c., Modern French âge) "age; life, lifetime, lifespan; maturity," earlier edage, from Vulgar Latin *aetaticum (source of Spanish edad, Italian eta, Portuguese idade "age"), from Latin aetatem (nominative aetas), "period of life, age, lifetime, years," from aevum "lifetime, eternity, age," from PIE root *aiw- "vital force, life, long life, eternity" (see eon). Meaning "time something has lived, particular length or stage of life" is from early 14c. Used especially for "old age" since early 14c. Expelled native eld.
age (v.): "to grow old," late 14c., from age (n.). Meaning "to make old" is early 15c. Related: Aged; aging.
age-group (n.): 1904, originally a term in the science of demographics, from age (n.) + group (n.).
age-old (adj.): 1896, from age (n.) + old.
aged (adj.): "having lived long," mid-15c., past participle adjective from age (v.). Meaning "having been allowed to get old" (of cheese, etc.) is by 1873. Meaning "of the age of" is from 1630s. Aged Parent is from "Great Expectations" (1860-61).
ageism (n.): "discrimination against people based on age," coined 1969 by U.S. gerontologist Dr. Robert N. Butler, from age + -ism, on pattern of racism, sexism. Related: Ageist.
ageless (adj.): 1650s, from age + -less. Related: Agelessly; agelessness.
agency (n.): 1650s, "active operation," from Medieval Latin agentia, noun of state from Latin agentem (nominative agens) "effective, powerful," present participle of agere "to drive, conduct" (see act (n.)). Meaning "establishment where business is done for another" first recorded 1861.
agenda (n.): 1650s, from Latin agenda, literally "things to be done," neuter plural of agendus, gerundive of agere "to do" (see act (n.)). Originally theological (opposed to matters of belief), sense of "items of business to be done at a meeting" first attested 1882. "If a singular is required (=one item of the agenda) it is now agendum, the former singular agend being obsolete" [Fowler].
agent (n.): late 15c., "one who acts," from Latin agentem (nominative agens) "effective, powerful," present participle of agere "to set in motion, drive, lead, conduct" (see act (n.)). Meaning "any natural force or substance which produces a phenomenon" is from 1550s. Meaning "deputy, representative" is from 1590s. Sense of "spy, secret agent" is attested by 1916.
agent (adj.): 1610s, from agent (n.).
Agent Orange (n.): powerful defoliant used by U.S. military in the Vietnam War, attested from 1971, said to have been used from 1961; so called from the color strip on the side of the container, which distinguished it from Agent Blue, Agent White, etc., other herbicides used by the U.S. military. Banned from April 1970.
aggie (n.1): "college student studying agriculture," by 1880, American English college slang, from agriculture + -ie.
aggie (n.2): type of toy marble, by 1905, American English, colloquial shortening of agate (q.v.).
Excited groups gather about rude circles scratched in the mud, and there is talk of "pureys," and "reals," and "aggies," and "commies," and "fen dubs!" There is a rich click about the bulging pockets of the boys, and every so often in school time something drops on the floor and rolls noisily across the room. When Miss Daniels asks: "Who did that?" the boys all look so astonished. Who did what pray tell? ["McClure's Magazine," May 1905]
agglomerate (v.): 1680s, from Latin agglomeratus, past participle of agglomerare "to wind or add onto a ball," from ad- "to" (see ad-) + glomerare "wind up in a ball," from glomus (genitive glomeris) "ball of yarn," from PIE root *glem-. Related: Agglomerated; agglomerating.
agglomeration (n.): 1774, "action of collecting in a mass," from Latin agglomerationem (nominative agglomeratio), noun of action from past participle stem of agglomerare (see agglomerate). In reference to a mass so formed, it is recorded from 1833.
agglutinate (v.): 1580s (from 1540s as a past participle adjective), from Latin agglutinatus, past participle of agglutinare (see agglutination). Related: Agglutinated; agglutinating.
agglutination (n.): 1540s, from Latin agglutinationem (nominative agglutinatio), noun of action from past participle stem of agglutinare "fasten with glue," from ad- "to" (see ad-) + glutinare "to glue," from gluten "glue," from PIE *glei- (see glue (n.)). Philological sense first recorded 1650s, in agglutinative.
agglutinative (adj.): 1630s, in a medical sense, from Latin agglutinat-, past participle stem of agglutinare (see agglutination). Philological sense is from 1650s.
aggrandisement (n.): chiefly British English spelling of aggrandizement. See -ize.
aggrandize (v.): 1630s, "to make larger, increase," from French agrandiss-, present participle stem of agrandir "to augment" (16c.), ultimately from Latin ad "to" (see ad-) + grandire "to make great," from grandis (see grand (adj.)). The double -g- spelling in English is by analogy with Latin words in ad-. Related: Aggrandized; aggrandizing.
aggrandizement (n.): 1650s, from French agrandissement, noun of action from agrandir (see aggrandize).
aggravate (v.): 1520s, "make heavy, burden down," from past participle adjective aggravate "burdened; threatened" (late 15c.), from Latin aggravatus, past participle of aggravare "to render more troublesome," literally "to make heavy" (see aggravation). Earlier in this sense was aggrege (late 14c.). Meaning "to make a bad thing worse" is from 1590s; that of "exasperate, annoy" is from 1610s.
To aggravate has properly only one meaning -- to make (an evil) worse or more serious. [Fowler]
Related: Aggravated; aggravating. Phrase aggravating circumstances is recorded from 1790.
aggravated (adj.): 1540s, "increased, magnified," past participle adjective from aggravate. Meaning "irritated" is from 1610s; that of "made worse" is from 1630s. The earlier adjective was simply aggravate (late 15c.).
aggravation (n.): late 15c., from Middle French aggravation, from Late Latin aggravationem (nominative aggravatio), noun of action from past participle stem of Latin aggravare "make heavier," figuratively "to embarrass further, increase in oppressiveness," from ad "to" (see ad-) + gravare "weigh down," from gravis "heavy" (see grave (adj.)). Oldest sense is "increasing in gravity or seriousness;" that of "irritation" is from 1610s.
aggregate (adj.): c. 1400, from Latin aggregatus "associated," literally "united in a flock," past participle of aggregare "add to (a flock), lead to a flock, bring together (in a flock)," from ad- "to" (see ad-) + gregare "herd" (see gregarious).
aggregate (v.): c. 1400, from Latin aggregatum, neuter past participle of aggregare (see aggregate (adj.)). Related: Aggregated; aggregating.
aggregate (n.): "number of persons, things, etc., regarded as a unit," early 15c., from noun use of Latin adjective aggregatum, neuter of aggregatus (see aggregate (adj.)).
aggregation (n.): early 15c., from Middle French agrégation or directly from Medieval Latin aggregationem (nominative aggregatio), noun of action from past participle stem of Latin aggregare (see aggregate (adj.)).
aggress (v.): "attack," 1714, back-formation from aggression, but used earlier with a sense of "approach" (1570s) and in this sense from French aggresser, from Late Latin aggressare, frequentative of Latin aggredi "to approach, attack." Related: Aggressed; aggressing.
aggression (n.): 1610s, "unprovoked attack," from French aggression (16c.), from Latin aggressionem (nominative aggressio) "a going to, an attack," noun of action from past participle stem of aggredi "to approach; attack," from ad- "to" (see ad-) + gradi (past participle gressus) "to step," from gradus "a step" (see grade (n.)). Psychological sense of "hostile or destructive behavior" first recorded 1912 in A.A. Brill's translation of Freud.
aggressive (adj.): 1791, from Latin aggress-, past participle stem of aggredi "to approach, attack" (see aggression) + -ive. In psychological use from 1913, first in translations of Freud. Related: Aggressively; aggressiveness.
aggressor (n.): 1670s, from Latin aggressor, agent noun from aggredi "to approach, attack" (see aggression).
aggrieve (v.): early 14c., from Old French agrever "make worse; become worse," from Latin aggravare "make heavier" (see aggravation). Related: Aggrieved; aggrieving.