Green’s Dictionary of Slang

definitely.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1968		E. O’Brien Love Object (1970) 94: ‘Are you leaving your curlers in, Dot, till it starts?’ Eithne Duggan asked her friend. ‘Oh def.,’ Doris O’Beirne said.
1970		L. Bangs in Creem Nov–Dec. n.p.: Morrison, def, does not get a pie in the face. He ’fessed up!
1989		P. Munro Sl. U.
1997		N. Barlay Curvy Lovebox 178: We’ll talk yeah? — Yeh yeh def.
2000		N. Barlay Crumple Zone You’d hurt him? — Yeah . . . Yeah Cee I would def.
2012		M. Lacher On the Bro’d 38: I asked the fat dude [...] if I could hang there and he was like ‘Def’.