(also avant garde, avantgarde); French, literally "advance guard" (see avant + guard (n.)). Used in English 15c.-18c. in a literal, military sense; borrowed again 1910 as an artistic term for "pioneers or innovators of a particular period." Also used around the same time in a political sense in communist and anarchist publications. As an adjective, by 1925.

The avant-garde générale, avant-garde stratégique, or avant-garde d'armée is a strong force (one, two, or three army corps) pushed out a day's march to the front, immediately behind the cavalry screen. Its mission is, vigorously to engage the enemy wherever he is found, and, by binding him, to ensure liberty of action in time and space for the main army. ["Sadowa," Gen. Henri Bonnal, transl. C.F. Atkinson, 1907]

1680s, a nautical interjection, "hold! stop!" probably worn down from Dutch houd vast "hold fast." See hold (v.) + fast (adv.).

AVAST. — The order to stop, or pause, in any exercise or operation; as Avast heaving — that is to say, desist, or stop, from drawing in the cable or hawser, by means of the capstan &c. [George Biddlecombe, "The Art of Rigging," 1848]

1784, "descent of a Hindu deity to earth in an incarnate or tangible form," from Sanskrit avatarana "descent" (of a deity to the earth in incarnate form), from ava- "off, down" (from PIE root *au- (2) "off, away") + base of tarati "(he) crosses over," from PIE root *tere- (2) "cross over, pass through, overcome."

Meaning "concrete embodiment of something abstract" is from 1815. In computer use, it seems to trace to the novel "Snowcrash" (1992) by Neal Stephenson.