Green’s Dictionary of Slang

(US) very, extremely, completely, a general intensifier.

1836		H.M. Milner Turpin’s Ride to York II vi: Yes, my Betty says, I be a main pretty lad.
1888		W.B. Churchward Blackbirding In The South Pacific 43: ‘Well my lad, how goes it?’ ‘Main bad,’ said I.
1907		S.E. White Arizona Nights 160: ‘That’s main queer,’ muttered Anderson.