Regional pronunciation of the noun "man," often in the American South -- though it appears frequently in Chicagoblues music, and particularly in the songs of Muddy Waters, where this pronunciation can be clearly heard.
I'm a mayne!
A full-grown mayne!
I'm a lovin' mayne!
-- Muddy Waters, "Mannish Boy"
Mayne is NOT a synonym for man. It is southern dialect slang way to PRONOUNCE man. You won't see it spelled "mayne" by the people who talk that way... except maybe in written lyrics or dialogue (such as in fiction) or for emplasis. Other than that, why would anyone go to the trouble to write a word that's almost twice as long as the correct one and not as widely understood?
I could call you my main mayne and it might sound clever in a movie but makes no sense in print.