Green’s Dictionary of Slang

(US) enormous, outstanding.

1970		T. Southern Blue Movie (1974) 9: He began his final laugh, his boss laugh, the kind that [...] turns into a monstro cough.
1995		J. Stahl Permanent Midnight 78: A monstro five-hour session listening to Mickey.
2015		J. Stahl OG Dad 88: The tiny faux-hawked nipple-sucker perched atop E’s monstro breast.