Green’s Dictionary of Slang

sure! excl.

also shure!
[coined in the UK, the term moved to the US by the mid-19C, although it returned to the UK in the early 20C+]

definitely! absolutely!

[UK]Fletcher Spanish Curate IV v: This fellow coins sure.
[UK]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.
[UK]O. Goldsmith ‘A Reverie at the Boar’s-head-tavern, Eastcheap’ in Coll. Works (1966) III 100: Sure the woman is dreaming, interrupted I.
[UK]Devizes & Wilts Gaz. 25 Dec. 4/1: Didn’t we live like fighting cocks sure!
[US]G.W. Harris ‘The Knob Dance’ Spirit of the Times (N.Y.) XV July in Inge (1967) 45: He would ‘See the Elephant,’ sure!
[US]F.M. Whitcher Widow Bedott Papers (1883) 41: O shaw! – stop yer hectorin’ about the Elder.
[Aus]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.
[US]Lantern (N.O.) 6 Aug. 5: Oh, this is a nerve, sure.
[UK]Boy’s Own Paper 4 May 484: Sure, thin, ye wouldn’t be after doubting the word av a gintleman born?
[US]‘O. Henry’ ‘The Gold that Glittered’ in Strictly Business (1915) 29: ‘Sure!’ he said with a grin.
[Ire]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.
[US]R. Chandler ‘Blackmailers Don’t Shoot’ in Red Wind (1946) 87: Mallory nodded, said: ‘Sure—for your own reasons.’.
[UK]Whizzbang Comics 11: Sure, I’ll try my luck.
[US]J. Thompson Criminal (1993) 9: Sure, I’ll do such and such a job for you.
[US]J. Mills Panic in Needle Park (1971) 28: ‘Sure,’ Bob told me, ‘hang around all you like. You’re a nice guy.’.
[UK](con. 1940s) J.G. Farrell Singapore Grip 173: Sure, I believe you.
[UK]Beano Comic Library No. 96 53: What? Sure! We’ll stop him.
[US]Tarantino & Avery Pulp Fiction [film script] 38: ‘Have you ever fantasised about being beaten up by a girl?’ ‘Sure.’.
[UK]Guardian G2 17 Mar. 7: Sure, they did a lot for this place.

In exclamations

sure, Mike!

(US) excl. of affirmation.

[US]Washington Bee (DC) 28 Aug. 8/1: ‘You cannot expect blood from a stone’ [...] ‘Sure Mike’.
[US]Indianapolis Jrnl (IN) 22 May 33/1: [cartoon caption] The Girl: ‘Dat ain’t de way to play we’re married.’ The Boy: ‘Sure, Mike! I’m coming home on a toot’.
Coshocton Dly Times (OH) 27 Aug. 8/7: Why, hadn’t he lived Here since ’84 and found that the Place was punky? Sure, Mike!
[US]Pensacola Jrnl (FL) 10 Jan. 7/2: [cartoon caption] Sure Mike! If d’ey ain’t lively den dey ain’t frogs!
[US]Laclede Blade (MO) 3 Dec. 1/4: ‘Do you have a better Hallmark watch than that twenty dollar one?’ ‘Sure Mike, we have’.
[US]N.Y. Tribune 24 Dec. 3/4: So what do we care? [...] turkey twice a year is all we’re getting out of it now. Sure, Mike.