Green’s Dictionary of Slang

(US teen/campus) completely, absolutely, very, exceedingly.

1982		M. Pond Valley Girl’s Guide to Life 52: It was mondo party time [...] All these foxy dudes.
1992		J. Mowry Way Past Cool 10: ‘Mondo gross,’ said Ric.
2002		‘Valley Girls’ on Paranoiafanzine 🌐 We’re the popular girls at school who are always into the mondo full buf and who everyone thinks are totally bitchen.
2011		J.J. DeCeglie Drawing Dead [ebook] He thought I was mondo drunk. Bourbon bombed.