Meaning of A- in English
a, as a prefix to english words, is derived from various sources. (1) it frequently signifies on or in (from an, a forms of ·as on), denoting a state, as in afoot, on foot, abed, amiss, asleep, aground, aloft, away (·as onweg), and analogically, ablaze, atremble, ·etc. (2) ·as of off, from, as in adown (·as ofd/ne off the dun or hill). (3) ·as a- (·goth. us-, ur-, ·ger. er-), usually giving an intensive force, and sometimes the sense of away, on, back, as in arise, abide, ago. (4) ·of y- or i- (corrupted from the ·as inseparable particle ge-, cognate with ohg. ga-, gi-, ·goth. ga-), which, as a prefix, made no essential addition to the meaning, as in aware. (5) ·fr. a (·lat. ad to), as in abase, achieve. (6) ·lat. a, ab, abs, from, as in avert. (7) greek insep. prefix / without, or privative, not, as in abyss, atheist; akin to ·eng. un-.
Webster English vocab. Английский словарь Webster. 2012