Green’s Dictionary of Slang

(US black) a term of address, a friend.

1952		W.R. Burnett Vanity Row 14: ‘Kit—you insured?’ [...] ‘You must be kidding, slim. You think I want somebody waiting for me to die?’.
1964		B. Jackson Get Your Ass in the Water (1974) 154: Now I wouldn’t be tryin’ to win myself no woman, / slim, I just like to hear them poor whores moan.
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Slim (noun & voc.) Person, friend.