def, adj. — Green’s Dictionary of Slang

def adj.

[? black pron. of SE death; or ? abbr. SE definitive. Note 1907 citation in Cassidy & LePage, Dict. of Jam. English (1967, 1992): ‘I never do him one def ting’, where def means ‘single’]

(orig. US black) perfect, excellent, first-rate.

1979		implied in def O.J. n.
1983		Kool Moe Dee ‘Ho Ya Like Me Now?’ 🎵 I’m bigger and better, forget about deffer.
1990		W.D. Myers Mouse Rap 66: He ran with a crew and you got to know / They were def with the games of long ago.
2002		‘Touré’ Portable Promised Land (ms.) 161: We Words (My Favorite Things) [...] Diss. Def. Down.
2011		Game ‘History’ 🎵 I’m universal, your deffest jam is a subsidiary.

In phrases

do it to def (v.)

(US) to do something as well as possible.

2001		Alliance [record title] Do It To Def.
2008		review at www.atypicalsole.com 🌐 Liva and Sandman dumb out all over this joint and you already know the Clipse did it to def.
2008		Busta Rhymes ‘Don’t Touch Me’ 🎵 Right to the left, do it to def.