a- -ing

The first element is a reduced form of Old English an (“on”); the second element is the Middle English gerund suffix -ing(e) from Old English -ung, -ing. Therefore, I go a-hunting = I go on (a) hunting/I go on a hunt. Due to confusion with the unrelated Middle English present participle ending -inge (alteration of -inde from Old English -ende, the use of a(n) preposition (which was fast evolving into a distinct prefix) was extended to present participles (and not merely restricted to preceding verbal nouns). With this development, the a- -ing circumfix emerged. Nowadays, in the few dialects of English (such as, notably the dialect of Smith Island, Virginia) which retain this circumfix, it is only circumfixed to words which function as part of a verb phrase; otherwise, -ing is suffixed.

• (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