Green’s Dictionary of Slang

(US) a native of Maine.

1837		N. Hawthorne Amer. Notebooks (1932) 10: The British have lately imprisoned a man who was sent to take the census; and the Maniacs are much excited on the subject.
1939		‘Miscellany’ in AS XIV:3 234/1: Names for natives of American places . . . Mainiac [DARE].
1968		in DARE.
1975		J. Gould Maine Lingo 175: Maineiac is more used by out-of-staters than by bona fide residents of the Pine Tree Precinct, but the latter are capable of tossing it off to describe themselves when it suits.
1992		in DARE file.
2002		Surfline 🌐 Ask any ‘Mainiac’ about winters in this part of the globe, and you’ll get an earful of tales of brutal ice storms and the kind of sub-zero, wind-driven cold that sucks the life-sustaining heat from the very marrow of your bones.