a- -ing

The a is a reduced form of the preposition on, which was used in Old English with nouns indicating actions or states (e.g. iċ wæs on huntoþe, “I was hunting”). This is the source of such words as asleep and alive, which also show reduction of on to a. In the Middle English period, the preposition came to be used with the present participles of verbs as well (formed in -ing), eventually coming to be perceived as a mere prefix.

• (General American, Received Pronunciation) IPA^(key): /ə- -ɪŋ/

a- -ing

1. (archaic) Used to form present participles and verbal nouns; and, (dialectal) circumfixed to words which function as part of a verb phrase (e.g. “he went a-hunting”)
   • 1951, I. Opie, P. Opie, The Oxford Dictionary of Nursery Rhymes, 2nd edition, Oxford University Press, published 1997, page 63:

     Bye, baby Bunting, / Father's gone a-hunting, / Mother's gone a-milking, / Sister's gone a-silking, / Brother's gone to buy a skin / To wrap the baby Bunting in.

• The common practice is to hyphenate this circumfix’s first element and the word it inflects (e.g.: “a-running”); unhyphenated spellings (e.g.: “abreaking”) are rarer, but nonetheless correct.

• em- -en
• en- -en
• y- -t

• Agin, Agni, Angi, Gain, Gina, NGIA, Nagi, Ngai, ag'in, agin, gain, gain-, gina, inga