circumfix（3）

Forms transitive verbs denoting an increase in, or intensification of, the quality denoted by the enclosed word.

(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”)

Prebilabial form of en- -en; i.e., occurring when the first element precedes a ‘b’, ‘p’, or ‘m’.