Green’s Dictionary of Slang

sure adv.

[sure! excl.]

a general intensifier, definitely, absolutely.

[US]D.S. Crumb ‘Dialect of Southeastern Missouri’ in DN II:v 332: sure, adv. Pronounced shore, as ‘shore fine’.
[US]J. London Smoke Bellew Pt 11 [Internet] This has the makin’s of a Jim-dandy suburb, an’ it sure looks like it’ll be some popular.
[US]H.L. Wilson Somewhere in Red Gap 54: The minute I get plumb sure mad I get wily.
[US]C. McKay Home to Harlem 211: She is sure some wonderful brown. [...] Now I sure does understand why Ray is so scornful of them easy ones.
[US]‘Boxcar Bertha’ Sister of the Road (1975) 33: The shacks are hostile, and the railroad dicks will glom you sure, unless you’re lucky.
[US] in J. Breslin Damon Runyon (1992) 39: The guy from Boston sure could handset type: he did a column in forty-three minutes.