Green’s Dictionary of Slang

1. absolute, certain.

1967		P. Thomas Down These Mean Streets (1970) 85: I couldn’t see them, but I had that for-sure feeling that it was me they had in their mouths.
1977		N.C. Heard When Shadows Fall 140: He’d been in a thousand similar situations with a thousand different women [...] and it was for sure that he’d left them with their two thousand thighs all gaped the same .
1984		Dly Oklahoman 19 May 78/1: If alcoholism were a diserease [...] there would be a clear-cut for-sure kind of test, like there is for other diseases.
2001		Indianapolis Star 9 May 40/3: In ‘The Real For-Sure Sory of Raggedy-Ann,’ the ragdoll sports hair of brown yarn.
2007		Calgary Herald (Alberta) 26 Oct. Rush Hour 4/1: She has another meeting early next year [...] ‘It’s not a for-sure, for-sure thing yet’.

2. reliable, trustworthy.

1994

UGK ‘Three Sixteens’ 🎵 And I’m down with some fa sho niggas.