Meaning of A in English
( prep. )
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.
Webster's Revised Unabridged English Dictionary. Пересмотренный полный словарь английского языка Webster. 2012