Online Etymology Dictionary

G: seventh letter of the alphabet, invented by the Romans; for its history see C. As a movie rating in the U.S., 1966, standing for general. Standing for gravity in physics since 1785.
g spot (n.): also g-spot, 1981, short for Gräfenberg spot, named for German gynecologist Ernst Gräfenberg (1881-1957), who described it in 1950.
G-man (n.): "FBI agent," 1930, shortening of government man; used earlier in an Irish context (1917), but the abbreviation is perhaps the same one.
G-string (n.): 1878, geestring, "loincloth worn by an American Indian," originally the string that holds it up, etymology unknown. The spelling with G (1882) is perhaps from influence of violin string tuned to a G (in this sense G string is first recorded 1831), the lowest and heaviest of the violin strings. First used of women's attire 1936, with reference to strip-teasers.
I AM the spirit of the silver "G":
I am silvered sadness,
I am moonlit gladness,
I am that fine madness
Of reverence half, and half of ecstasy
[from "Spirit of the 'G' String," Alfred L. Donaldson, in "Songs of My Violin," 1901]
G.A.R.: 1867, abbreviation of Grand Army of the Republic, the organization founded by union veterans of the American Civil War. The Grand Army was the name given (on the French model) to the army that organized in Washington in 1861 to put down the rebellion.
G.I. (adj.): also GI, 1936 as an adjective meaning "U.S. Army equipment," American English, apparently an abbreviation of Government Issue, and applied to anything associated with servicemen. Transferred noun sense of "U.S. Army soldier" arose during World War II (first recorded 1943), apparently from the jocular notion that the men themselves were manufactured by the government.

An earlier G.I. (1908) was an abbreviation of galvanized iron, especially in G.I. can, a type of metal trash can; the term was picked up by U.S. soldiers in World War I as slang for a similar-looking type of German artillery shells. But it is highly unlikely that this G.I. came to mean "soldier." No two sources seem to agree on the entire etymology, but none backs the widespread notion that it stands for *General Infantry. GI Joe "any U.S. soldier" attested from 1942 (date in OED is a typo).
gab (v.): "talk much," 1786, probably via Scottish and northern England dialect from earlier sense "speak foolishly; talk indiscreetly" (late 14c.), from gabben "to scoff, jeer; mock (someone), ridicule; reproach (oneself)," also "to lie to" (late 13c.), from a Scandinavian source, such as Old Norse gabba "to mock, make fun of," and probably in part from Old French gaber "to mock, jest; brag, boast," which, too, is from Scandinavian. Ultimately perhaps imitative (compare gabble, which might have shaded the sense of this word). Gabber was Middle English for "liar, deceiver; mocker." Related: Gabbed; gabbing.
gab (n.): "action of talking," earlier "chatter, loquacity, idle talk" (mid-13c.), also "falsehood, deceit," originally "a gibe, a taunt" (c. 1200), mid-13c., probably from a Scandinavian source akin to Old Norse gabb "mocking, mockery," and in part from Old French gap, gab "joke, jest; bragging talk," which also is probably from Scandinavian (compare gab (v.)). Probably also there is influence from Scottish and northern English gab "the mouth" (see gob); OED reports the word "Not in dignified use." Gift of (the) gab "talent for speaking" is from 1680s.
gabardine (n.): 1590s, "dress, covering," variant of gaberdine. Meaning "closely woven cloth" is from 1904.
gabble (v.): "to talk noisily, rapidly, and incoherently," 1570s, frequentative of gab (v.), or else imitative. Related: Gabbled; gabbling.
gabble (n.): "senseless, loud, rapid talk; animal noise," c. 1600, from gabble (v.).
gabbro (n.): type of igneous rock, 1823, introduced in geology 1809 by German geologist Christian Leopold von Buch (1774-1853), from Italian (Tuscan) gabbro, a word among the marble-workers, of obscure origin; perhaps from Latin glaber "bare, smooth, bald" (see glad). Related: Gabbroic.
gabby (adj.): "garrulous, talkative," 1719, originally Scottish, from gab (n.) + -y (2). Related: Gabbiness.
Gabe: masc. proper name, usually short for Gabriel.
gaberdine (n.): "long, loose outer garment," 1510s, from Spanish gabardina, which Watkins says is from Middle French galverdine, from a Germanic source such as Middle High German wallevart "pilgrimage" (German Wallfahrt) in the sense of "pilgrim's cloak." The compound would represent Proto-Germanic *wal- (source also of Old High German wallon "to roam, wander, go on a pilgrimage;" see gallant (adj.)) and Proto-Germanic *faran "to go" (see fare (v.)). The Spanish form perhaps was influenced by Spanish gabán "overcoat" and tabardina "coarse coat." Century Dictionary, however, says the Spanish word is an extended form of gabán and the Spanish word was borrowed and underwent alterations in Old French.
gabfest (n.): "session of conversation," 1895, American English colloquial, from gab + -fest.
gable (n.): "end of a ridged roof cut off in a vertical plane, together with the wall from the level of the eaves to the apex," mid-14c., "a gable of a building; a facade," from Old French gable "facade, front, gable," from Old Norse gafl "gable, gable-end" (in north of England, the word probably is directly from Norse), according to Watkins, probably from Proto-Germanic *gablaz "top of a pitched roof" (source also of Middle Dutch ghevel, Dutch gevel, Old High German gibil, German Giebel, Gothic gibla "gable"). This is traced to a PIE *ghebh-el- "head," which seems to have yielded words meaning both "fork" (such as Old English gafol, geafel, Old Saxon gafala, Dutch gaffel, Old High German gabala "pitchfork," German Gabel "fork;" Old Irish gabul "forked twig") and "head" (such as Old High German gibilla, Old Saxon gibillia "skull").
Possibly the primitive meaning of the words may have been 'top', 'vertex'; this may have given rise to the sense of 'gable', and this latter to the sense of 'fork', a gable being originally formed by two pieces of timber crossed at the top supporting the end of the roof-tree. [OED]
Related: Gabled; gables; gable-end.
Gabriel: masc. proper name, also name of an Old Testament angel, from Hebrew Gabhri el, literally "man of God," from gebher "man" + El "God." First element is from base of verb gabhar "was strong" (compare Arabic jabr "strong, young man;" jabbar "tyrant"). Gabriel's hounds (17c.) was a folk explanation for the cacophony of wild geese flying over, hidden by clouds or night.
gad (v.): mid-15c., gadden, "go quickly, hurry," of uncertain origin, perhaps from gad (n.) "sharp stick for driving oxen" on the notion of moving as animals do when being driven by a gad. There also was a Middle English gadeling (Old English gædeling) "kinsman, companion in arms; fellow, man," but which had a deteriorated sense of "person of low birth, rogue, vagabond" by c. 1300 (it also had a meaning "wandering," but this is attested only from 16c.). Related: Gadding.
gad (n.): c. 1300, "a goad, sharp pointed stick to drive oxen, etc.;" c. 1400, "sharp-pointed metal spike," from Old Norse gaddr "spike, nail," from Proto-Germanic *gadaz "pointed stick" (see yard (n.2)). Attested earlier as "metal bar or rod, ingot" (mid-13c.) hence also in Middle English a unit of length in land-measure, varying from 10 to 16 feet. Not related to goad (n.), but perhaps influenced by it in sense.
gadabout (n.): "one who gads or walks idly about, especially from motives of curiosity or gossip" [Century Dictionary], 1830; see gad (v.) + about (adv.). As an adjective from 1817. Verbal phrase gadder about is attested from 1560s.
Gadarene swine (n.): an image from Matt. viii:28. From Gadara, town of ancient times near Galilee.
gadfly (n.): also gad-fly, 1620s, "fly which bites cattle," probably from gad (n.) "goad, metal rod," here in the sense of "stinger;" but the sense is entangled with gad (v.) "rove about" (on the notion, perhaps, of the insect's power of flight or of the restlessness of animals plagued by them), and another early meaning of gadfly was "someone who likes to go about, often stopping here and there" (1610s). Sense of "one who irritates another" is from 1640s (equivalent of Latin oestrus; see estrus). "In strictness, only the females are gadflies, the males being smaller and quite inoffensive, living on juices of plants" [Century Dictionary]. Earlier bot-fly, from bot "skin parasite" (late 15c.).
gadget (n.): 1886, gadjet (but said by OED corespondents to date from 1850s), sailors' slang word for any small mechanical thing or part of a ship for which they lacked, or forgot, a name; perhaps from French gâchette "catch-piece of a mechanism" (15c.), diminutive of gâche "staple of a lock." OED says derivation from gauge is "improbable."
Gadhelic (adj.): 1796, originally "Irish," now "pertaining to the Gaels" in the broadest sense; a "discriminated form" [Century Dictionary] of Gaelic.
gadolinium (n.): metallic element, with element ending -ium + gadolinia, an earth named 1886 by J.C. Marginac in honor of Johan Gadolin (1760-1852), Finnish mineralogist and chemist, who in 1794 first began investigation of the earth (subsequently called gadolinite, 1802) which eventually yielded this element and several others.
gadzooks (interj.): 1690s, a condensed form of some exclamation, usually said to be God's hooks (nails of Christ's Cross) or even God's hocks. Compare godsookers (1670s). The use of Gad for God (as in egad) is first attested 1590s. Among other similar "phraseological combinations" noted by OED (all from 17c.) were gadsbobs, gadslid, gadsniggers, gadsbudlikins, and gadsnouns; in all of which the second elements are sometimes said to be mere fanciful syllables.
Gaea: see Gaia.
Gael (n.): 1810, from Scottish Gaelic Gaidheal "member of the Gaelic race" (Irish, Scottish, Manx), corresponding to Old Irish Goidhel (compare Latin Gallus under Gallic, also see Galitia, Galatians). The native name in both Ireland and Scotland; owing to the influence of Scottish writers Gael was used in English at first exclusively of Highland Scots.
Gaelic (adj.): 1774, "of or pertaining to the Gaels" (meaning originally in English the Scottish Highlanders); 1775 as a noun, "language of the Celts of the Scottish Highlands;" earlier Gathelik (1590s), from Gael (Scottish Gaidheal; see Gael) + -ic.
gaff (n.3): "cheap music hall or theater; place of amusement for the lowest classes," 1812, British slang, earlier "a fair" (1753), of unknown origin.
gaff (n.1): "iron hook," c. 1300, gaffe, from Old French gaffe "boat hook" (see gaffe). Specifically of the hook on a fishing spear from 1650s. As a type of spar from 1769. Related: gaff-hook.
gaff (n.2): "talk," 1812, in phrase blow the gaff "spill a secret," of uncertain origin. OED points out Old English gafspræc "blasphemous or ribald speech," and Scottish gaff "loud, rude talk" (by 1825). Compare gaffe.
gaffe (n.): "blunder," 1909, perhaps from French gaffe "clumsy remark," originally "boat hook," from Middle French gaffe (15c.), from Old Provençal gafar "to seize," probably from a Germanic source, from Proto-Germanic *gaf-, which is perhaps from PIE *kap- "to grasp, catch" (see capable). Sense connection between the hook and the blunder is obscure; the gaff was used to land big fish. Or the Modern English word might derive from British slang verb gaff "to cheat, trick" (1893); or gaff "criticism" (1896), from Scottish dialect sense of "loud, rude talk" (see gaff (n.2)).
gaffer (n.): 1580s, "elderly rustic," apparently (based on continental analogies) a contraction of godfather (compare gammer). Originally a term of respect, also applied familiarly; from "old man" it was extended by 1841 to foremen and supervisors, which sense carried over in early 20c. to "electrician in charge of lighting on a film set."
gag (v.): mid-15c., transitive, "to choke, strangle" (someone), possibly imitative and perhaps influenced by Old Norse gag-hals "with head thrown back." The sense of "stop a person's mouth by thrusting something into it" is first attested c. 1500. Intransitive sense of "to retch" is from 1707. Transitive meaning "cause to heave with nausea" is from 1945. Related: Gagged; gagging.
gag (n.2): "a joke," 1863, especially a practical joke, probably related to theatrical sense of "matter interpolated in a written piece by the actor" (1847); or from the sense "made-up story" (1805); or from slang verbal sense of "to deceive, take in with talk" (1777), all of which perhaps are from gag (v.) on the notion of "to stuff, fill" (see gag (v.)). Gagster "comedian" is by 1932.
gag (n.1): "something thrust into the mouth or throat to prevent speaking," 1550s, from gag (v.); figurative use, "violent or authoritative repression of speech," is from 1620s. Gag-law in reference to curbs on freedom of the press is from 1798, American English. The gag-rule that blocked anti-slavery petitions in the U.S. House of Representatives was in force from 1836 to 1844.
gaga (adj.): "crazy, silly," 1920, probably from French gaga "senile, foolish," probably imitative of meaningless babbling.
gage (n.): "a pledge, a pawn, something valuable deposited to insure performance," especially "something thrown down as a token of challenge," c. 1300, from Old French gage "pledge (of battle), security, guarantee; pay, reward" (11c.), from Frankish *wadja-, from Proto-Germanic *wadi- (see wed). Italian gaggio, Spanish and Portuguese gage are French loan-words.
gage (v.): c. 1400, "to deposit as security," from Old French gager, gagier "to guarantee, promise, pledge, swear; bet, wager; pay," from gage "security, pledge" (see gage (n.)). Related: Gaged; gaging. For the measuring sense, see gauge (v.).
gaggle (n.): late 15c., gagyll, with reference to both geese and women (on the notion of "chattering company"). Barnhart says possibly from Old Norse gagl "small goose, gosling, wild goose;" OED calls it "one of the many artificial terms invented in the 15th c. as distinctive collectives referring to particular animals or classes of persons." Possibly of imitative origin (compare Dutch gagelen "to chatter;" Middle English gaggle "to cackle," used of geese, attested from late 14c.). The loosened general sense of "group of people" is from 1946.
Gaia (n.): Earth as a goddess, from Greek Gaia, spouse of Uranus, mother of the Titans, personification of gaia "earth" (as opposed to heaven), "land" (as opposed to sea), "a land, country, soil;" it is a collateral form of ge (Dorian ga) "earth," which is of unknown origin and perhaps from a pre-Indo-European language of Greece. The Roman equivalent goddess of the earth was Tellus (see tellurian), sometimes used in English poetically or rhetorically for "Earth personified" or "the Earth as a planet."
gaiety (n.): "cheerfulness, mirth," 1630s, from French gaieté (Old French gaiete, 12c.), from gai "gay" (see gay). In the 1890s, in Britain, especially with reference to a London theater of that name, and the kind of musical shows and dancing girls it presented.
Gail: fem. proper name, in some cases short for Abigail, or from the Hebrew root in that name meaning "rejoicing." Attained its greatest popularity in U.S. as a given name for girls born c. 1945-1955.
gaily (adj.): also gayly, "with mirth and frolic," late 14c., from Middle English gai (see gay) + -ly (2). "The spelling gaily is the more common, and is supported by the only existing analogy, that of daily" [OED].
gain (n.): late 15c., "that which has been acquired" (possessions, resources, wealth), from Middle French gain, from Old French gaaigne "gain, profit, advantage; work, business; booty; arable land" (12c.), from gaaignier "to gain, earn; capture, win" (see gain (v.)). Meaning "any incremental increase" (in weight, etc.) is by 1851. Related: Gains.

The original French word enfolded the notions of "profit from agriculture" and "booty, prey." Neither the verb nor the noun gain is in Middle English, which however had gainage "profit derived from agriculture" (late 14c., from Old French gaaignage); gaineier "farmer" (late 13c. as a surname); gainerie "a farm" (mid-15c.).
gain (v.): 1520s, "obtain as profit," from Middle French gagner, from Old French gaaignier "to earn, gain; trade; capture, win," also "work in the fields, cultivate land," from Frankish *waidanjan "hunt, forage," also "graze, pasture," from Proto-Germanic *waithanjan "to hunt, plunder," from *waithjo- "pursuit, hunting" (source also of Old English waþ "hunting," German Weide "pasture, pasturage," Old Norse veiðr "hunting, fishing, catch of fish").

This is from PIE root *weie- "to go after, strive after, pursue vigorously, desire," with noun derivatives indicating "force, power" (related to *wi-ro- "man;" see virile). Cognates include Sanskrit padavi- "track, path, trail," veti- "follows, strives, leads, drives;" Avestan vateiti "follows, hunts;" Greek hiemai "move oneself forward, strive, desire;" Lithuanian vyti "to chase, pursue;" Old Norse veiðr "chase, hunting, fishing;" Old English OE wað "a chase, hunt."

Meaning "obtain by effort or striving" is from 1540s; intransitive sense of "profit, make gain" is from 1570s. Meaning "arrive at" is from c. 1600. Of timepieces by 1861. Related: Gained; gaining. To gain on "advance nearer" is from 1719. To gain ground (1620s) was originally military.
gainer (n.): "one who gains or profits," 1530s, agent noun from gain (v.). As "one who (deliberately) gains weight" by 2000s.
gainful (adj.): "producing profit or advantage," 1550s, from gain (n.) + -ful. Phrase gainfully employed attested from 1796. Related: Gainfully (1540s).