Green’s Dictionary of Slang

certainly! definitely! absolutely!

1890		Record-Union (Sacramento, CA) 29 Nov. 7/1: ‘You’ve been a soldier,’ he said. ‘For sure, me bhoy!’.
1911		Philadelphia Inquirer (PA) 2 Apr. 60/2: ‘For sure [...] ’tis all along of his flirtatious ways the trouble has come about’.
1917		‘A-No. 1’ From Coast to Coast with Jack London 38: For sure! They are the lads who tried to burglarize my home, Mr. Officer!
1935		(con. late 1920s) L. Hughes Little Ham Act II: tiny: Gussie’s a troublemaker. Always talking. opal: For sure! lulu: Yes, she’s too broadcast.
1949		P. Pringle Boy’s Book of Cricket 69: One, for sure, sir!
1965		F.J. Hardy Yarns of Billy Borker 64: Did they sell all the tickets? For sure!
1977		C. McFadden Serial 13: ‘For sure’, she said forcefully.
1982		Frank Zappa ‘Valley Girl’ 🎵 Fer sure, fer sure / She’s a Valley Girl / So sweet ’n pure / Okay, fine . . . / Fer sure, fer sure.
1999		K. Sampson Powder 435: Oh, for sure, mate! That was part of it.
2016		Green bay Press-Gaz. (WI) 2 jan. C3/1: ‘Yeah, for sure,’ Rodgers said when asked if he wants the Packers to resign Jones.