Green’s Dictionary of Slang

1. a general intensifier, definitely, absolutely.

1754		Trial of Elizabeth Canning in Howell State Trials (1816) 483: After you missed your daughter [...] you took all the means in your power to know what was become of her? —Sure I did, I went to all the agents and places where I could think of, fearing some casualty.
1903		D.S. Crumb ‘Dialect of Southeastern Missouri’ in DN II:v 332: sure, adv. Pronounced shore, as ‘shore fine’.
1912		J. London Smoke Bellew Pt 11 🌐 This has the makin’s of a Jim-dandy suburb, an’ it sure looks like it’ll be some popular.
1916		H.L. Wilson Somewhere in Red Gap 54: The minute I get plumb sure mad I get wily.
1928		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.
1937		‘Boxcar Bertha’ Sister of the Road (1975) 33: The shacks are hostile, and the railroad dicks will glom you sure, unless you’re lucky.
1991		in J. Breslin Damon Runyon (1992) 39: The guy from Boston sure could handset type: he did a column in forty-three minutes.

2. as sense 1 but used ironically/dismissively cf. right phr.).

1992		P. Earley Hot House 195: ‘I’ll look into it,’ said [Warden] Matthews [...] ‘Sure,’ Silverstein replied. He didn’t believe him.
2001		W.D. Myers Bad Boy 172: One of the others told me to stay where I was while they went to get a gun. Sure.
2022		K. Noem Not My First Rodeo 54: We all started laughing. Sure, you did, Dad, we thought. Sure, you did.