Green’s Dictionary of Slang

(US) definitely, certainly, absolutely.

1902		F. Norris Pit (1994) 303: The broker glanced up incredulously. ‘Now you are for sure crazy.’.
1906		A.H. Lewis Confessions of a Detective 130: Ay! I’ve a story for sure.
1921		Eve. News (Harrisburg PA) 8 Mar. n.p.: Dicky Meets a Real-For-Sure Scarecrow.
1926		Van Vechten Nigger Heaven 145: Makes me feel I’m your daddy! / Honey, you is, fo’ sho’.
1938		X. Herbert Capricornia (1939) 106: You said he’d come for sure. Isn’t he going to take me?
1940		F. Hunt Long Trail from Texas 50: That’s the right time o’ year to have hell a-poppin’. An’ what those them mossy horns ’ll do in a real-fer-sure storm’ll be a caution.
1957		R. Prather Always Leave ’Em Dying 94: It would help if you could check with those two guards and find out for sure who sapped me.
1969		‘Iceberg Slim’ Pimp 39: He was a fox for sure.
1986		R. Campbell In La-La Land We Trust (1999) 5: She was self-made, she often said, For sure, no man had ever made her.
1990		S. Morgan Homeboy 4: Rings fur shur knew she had a career.
2000		C. Cook Robbers (2001) 131: Thinking the gal had the lingo down, that was for sure.
2011		N. Johnson ‘Corpse by Any Other Name’ in Pulp Ink [ebook] That is for sure a pile of dirt.
2016		Baltimore Sun (MD) 28 Oct. A17/1: Time is Money [...] ‘For sure I will use it [i.e. a superfast train],’ Hiroishi said.