Towards, of, in, into or at; marking some ongoing process or state; movement onwards or away.
[Old English prepositions of or on (sometimes as unstressed an), or the Old English prefix a-.]
The Old English prepositions were originally separate words, but became reduced to a- and attached to the words they once modified. The process can be seen in alive, which in Old English was two words, on līfe, literally ‘in life’; others of similar type are aside, akin, and anew. Some examples are verbs derived from Old English a-, which had an idea about it of an action or an intensification of an action: arise, abide, and awake.
Some adjectives imply a continuing or active state, and have much the same force as a present participle ending in -ing (see -ing2): ablaze, abuzz, afire, afoot, aglow, astride. Others combine the prefix with a present participle, usually hyphenated; such words imply an ongoing process or activity: a-brewing, a-roving, a-hunting, a-wasting; though they are mostly now archaic, literary, or dialectal, the form has had a small revival in recent decades, as in Bob Dylan's song lyric The times they are a-changing.