Green’s Dictionary of Slang

(US black) most definitely.

2000		Ebonics Primer at www.dolemite.com 🌐 mos def Definition: most definately Example: Keep it on da real yo!... mos def.
2002		Simon & Burns ‘The Target’ Wire ep. 1 [TV script] ‘That shit costs time and money. You gonna make that right?’ ‘Mos def.’.
2010		A. Steinberg Running the Books 270: I never seen a deer in my life and mos def not in the city.