Green’s Dictionary of Slang

an affirmative excl., absolutely! certainly! I agree!

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 113/1: ‘Sure, thin,’ said a young Irishman – he was a nice ’cute fellow – ‘sure, thin, ma’am, I should be afther offering you a taste.’.
1898		F.P. Dunne Mr Dooley in Peace and War 135: ‘Sure thing,’ said Mr. Dooley, winking at Mr. McKenna.
1901		‘Lawrence Lynch’ High Stakes 61: ‘Do we visit the rooms above?’ ‘Sure thing!’.
1913		F. Klaeber ‘A Word-List From Minnesota’ in DN IV:i 12: sure-shootin’, adv. Certainly. ‘Are you going to the ball game?’ ‘Sure-shootin’.’.
1916		P. Kyne Cappy Ricks 198: ‘You don’t mean to tell me sir, that you’ve given the Quickstep to that rowdy Peasley?’ Cappy relapsed into the colloquialism of the younger generation [...] ‘Surest thing you know,’ he said.
1917		A. Conan Doyle His Last Bow in Baring-Gould (1968) II 798: ‘I believe they are not far off me.’ ‘You don’t mean that!’ ‘Sure thing.’.
1923		‘Sapper’ Jim Maitland (1953) 112: ‘Sure thing, Mr. Maitland,’ answered the other.
1932		M. Marshall Travels of Tramp-Royal 229: Sure thing, Scottie. Lemme see – it was in Hackney Wick spike, wasn’t it?
1940		F. Sargeson ‘The Making of a New Zealander’ in A Man And His Wife (1944) 13: Yes, I said. Sure thing.
1954		Oh Boy! No. 16 11: Sure t’ing, Boss.
1965		(con. 1930s) R. Wright Lawd Today 191: ‘How about some cigars?’ asked Jake. ‘Sure thing.’.
1978		P. Theroux Picture Palace 184: ‘Sure thing,’ she said.
1985		A. Payne ‘Minder on the Orient Express’ Minder [TV script] 14: dryden: Your stepmother has no interest in the bequest. nikki: Sure thing.
2016		V. McDermid Out of Bounds (2017) 64: He nodded and smiled. ‘Sure thing, boss’.