Green’s Dictionary of Slang

1. definitely! absolutely!

1622		Fletcher Spanish Curate IV v: This fellow coins sure.
1728		Vanbrugh & Cibber Provoked Husband II i: You! you think I’m too forward! sure! Brother Mud! your Head’s too heavy to think of anything but your Belly.
1760		O. Goldsmith ‘A Reverie at the Boar’s-head-tavern, Eastcheap’ in Coll. Works (1966) III 100: Sure the woman is dreaming, interrupted I.
1823		Devizes & Wilts Gaz. 25 Dec. 4/1: Didn’t we live like fighting cocks sure!
1845		G.W. Harris ‘The Knob Dance’ Spirit of the Times (N.Y.) XV July in Inge (1967) 45: He would ‘See the Elephant,’ sure!
c.1850		F.M. Whitcher Widow Bedott Papers (1883) 41: O shaw! – stop yer hectorin’ about the Elder.
1876		S. James Vagabond Papers (1st series) 27: He was a blackguard. Shure, why didn’t he go and hit up policeman o Prince’s-bridge if he wanted to be locked up.
1887		Lantern (N.O.) 6 Aug. 5: Oh, this is a nerve, sure.
1901		Boy’s Own Paper 4 May 484: Sure, thin, ye wouldn’t be after doubting the word av a gintleman born?
1910		‘O. Henry’ ‘The Gold that Glittered’ in Strictly Business (1915) 29: ‘Sure!’ he said with a grin.
1922		Joyce Ulysses 293: And sure, more to be token, the lout I’m told was in Power’s after, the blender’s, round in Cope Street going home footless in a cab five times in the week after drinking his way through all the samples of the bloody establishment.
1933		R. Chandler ‘Blackmailers Don’t Shoot’ in Red Wind (1946) 87: Mallory nodded, said: ‘Sure—for your own reasons.’.
1942		Whizzbang Comics 11: Sure, I’ll try my luck.
1953		J. Thompson Criminal (1993) 9: Sure, I’ll do such and such a job for you.
1966		J. Mills Panic in Needle Park (1971) 28: ‘Sure,’ Bob told me, ‘hang around all you like. You’re a nice guy.’.
1978		(con. 1940s) J.G. Farrell Singapore Grip 173: Sure, I believe you.
1986		Beano Comic Library No. 96 53: What? Sure! We’ll stop him.
1993		Tarantino & Avery Pulp Fiction [film script] 38: ‘Have you ever fantasised about being beaten up by a girl?’ ‘Sure.’.
2000		Guardian G2 17 Mar. 7: Sure, they did a lot for this place.

2. used ironically, sarcastically.

1970		J. Bouton Ball Four 190: [L]ocal talent, moaning, says, ‘Oh darling, I’ve never done it that way before’ Whereupon Gosger sticks his head out, drawls, ‘Yeah, surrre,’ and retreats into the closet.
1973		P. Maas Serpico 183: Durk said he told Fraiman that he knew just the police officer who could help, and Fraiman was anxious to meet him. ‘Sure, Durk,’ Serpico said.
2008		T. Wolff ‘A White Bible’ in Our Story Begins 295: ‘Hassan’s English is fine. He’s lazy and dishonest, that’s his problem [...] ’ ‘Hassan is going to be a doctor’ ‘Sure’.