Meaning of A in English
prep. (usu. as prefix) 1 to, towards (ashore; aside). 2 (with verb in pres. part. or infin.) in the process of; in a specified state ( a-hunting; a-wandering; abuzz; aflutter). 3 on (afire; afoot). 4 in (nowadays).
[ weakening of OE prep. an, on (see ON) ]
Concise Oxford English dictionary. Краткий оксфордский словарь английского языка. 2012