Yes, you're right about "goin" (monosyllabic) as a source. Labials & velars in SW English project a /w/ after the consonant, so you get "bwile" for boil and "bwy" for boy. Both of these occur in Caribbean creoles. I don't know if either made it to these shores--bile sounds more usual in American vernaculars to me than bwile does. Yours, Paul Johnston