Green’s Dictionary of Slang

(US Hisp.) used in direct address, man.

1967		P. Thomas Down These Mean Streets (1970) 52: Macho, their president, jumped stink and said, ‘Time man, we got heart, we deal with our manos.’.
1967		J. Rechy Numbers (1968) 33: ‘Oh, gee, mano,’ using the hip Mexican appellation, ‘I knew you weren’t the paying kind.’.
1972		B. Rodgers Queens’ Vernacular.