4. a- a verbal prefix with the historical sense “out, up,” occurring in verbs and verb derivatives inherited from Old and Middle English, usu. marking the inception or completion of the action denoted by the base verb: abide; accursed; arise; ashamed; awake.
5. a- var. of ab - before b, m, and v: amanuensis; avert.
6. a- var. of ad- , used before sc, sp, st (ascend) and in words of French derivation, often with the sense of increase or addition (amass).
7. a- var. of an-1 before a consonant: amoral; atonal; achromatic.
Definition of 'a-'
Webster Dictionary
1. a- 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) Old English y- or i- (corrupted from the AS. inseparable particle ge-, cognate with OHG. ga-, gi-, Goth. ga-), which, as a prefix, made no essentialaddition to the meaning, as in aware. (5) French a (L. ad to), as in abase, achieve. (6) L. a, ab, abs, from, as in avert. (7) Greek insep. prefix / without, or privative, not, as in abyss, atheist; akin to E. un-
Definitions of a- 
