Webster's Revised Unabridged Dictionary -- Volume AB

2. [Originally the preposition a (an, on).] In each; to or for each; as, "twenty leagues a day", "a hundred pounds a year", "a dollar a yard", etc.

A (), prep. [Abbreviated form of an (AS. on). See On.]

1. In; on; at; by. [Obs.] "A God's name." "Torn a pieces." "Stand a tiptoe." "A Sundays" Shak. "Wit that men have now a days." Chaucer. "Set them a work." Robynson (More's Utopia).

2. In process of; in the act of; into; to; -- used with verbal substantives in -ing which begin with a consonant. This is a shortened form of the preposition an (which was used before the vowel sound); as in a hunting, a building, a begging. "Jacob, when he was a dying" Heb. xi. 21. "We'll a birding together." " It was a doing." Shak. "He burst out a laughing." Macaulay. The hyphen may be used to connect a with the verbal substantive (as, a-hunting, a-building) or the words may be written separately. This form of expression is now for the most part obsolete, the a being omitted and the verbal substantive treated as a participle.

A. [From AS. of off, from. See Of.] Of. [Obs.] "The name of John a Gaunt." "What time a day is it ?" Shak. "It's six a clock." B. Jonson.

A. A barbarous corruption of have, of he, and sometimes of it and of they. "So would I a done" "A brushes his hat." Shak.

A. An expletive, void of sense, to fill up the meter

A merry heart goes all the day,
Your sad tires in a mile-a.
Shak.

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. ofdūne off the dun or hill). (3) AS. ā- (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 essential addition to the meaning, as in aware. (5) French à (L. ad to), as in abase, achieve. (6) L. a, ab, abs, from, as in avert. (7) Greek insep. prefix α without, or privative, not, as in abyss, atheist; akin to E. un-.