a- (1) Look up a- at Dictionary.com
in native (derived from Old English) words, it most commonly represents Old English an "on" (see a (2)), as in alive, asleep, abroad, afoot, etc., forming adjectives and adverbs from nouns; but it also can be Middle English of, as in anew, abreast (1590s); or a reduced form of Old English past participle prefix ge-, as in aware; or the Old English intensive a-, as in arise, awake, ashame, marking a verb as momentary, a single event. In words from Romanic languages, often it represents Latin 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]
a- (2) Look up a- at Dictionary.com
prefix meaning "not," from Latin a-, short for ab "away from" (as in avert), or its cognate, Greek a-, short for apo "away from, from," both cognate with Sanskrit apa "away from," Gothic af, Old English of (see apo-).
a- (3) Look up a- at Dictionary.com
prefix meaning "not," from Greek a-, an- "not," from PIE root *ne "not" (see un-).