in native (derived from O.E.) words, it most commonly represents O.E. an "on" (see a
(2)), as in alive, asleep, abroad, ashore, etc., forming adjectives and adverbs from nouns; but it also can be M.E. of, as in anew, abreast (1590s); or a reduced form of O.E. pp. prefix ge-, as
in aware; or the O.E. intens. a-, as in arise, awake, ashame, marking a verb as momentary, a single event. In words from Romanic languages, often it represents L. ad- "to, at."
"[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose." [OED]
prefix meaning "not," from L. a-, short for ab "away from" (cf. avert), or its cognate, Gk. a-, short for apo "away from, from," both cognate with Skt. apa "away from," Goth. af, O.E. of.
prefix meaning "not," from Gk. a-, an- "not," from PIE base *ne "not" (see un-