Green’s Dictionary of Slang

(US) a general attrib. adj. of approval: genuine, qualified, responsible, trustworthy.

1841		Sthn Pioneer (Carrolton, MS) 20 Feb. 1/5: Major Bluster [...] was a ‘sure enough’ major who had fought [...] in the last war.
1861		letter q. in Wiley Life of Johnny Reb (1943) 272: [A] shore-nuff Sweetheart [...] with $50 thousand dollars, a lady of fine sence and education.
1884		J. Harrison ‘Negro English’ in Anglia VII 263: To year sho’ ’nuff cussin’ = to hear passionate words.
1899		F.E. Daniel Recollections of a Rebel Surgeon 232: No, he wasn’t a shonuff ‘colonel,’ you know.
1907		C.E. Mulford Bar-20 i: That’s shore nuff gospel, Thomp.
1910		Wodehouse Gentleman of Leisure Ch. xviii: I could pick out a fly cop from a bunch of a t’ousand. He’s a sure ’nough sleut’ all right.
1911		C.E. Mulford Bar-20 Days 89: Bet they think I’m a real, shore-’nuff medicine-man.
1913		H.A. Franck Zone Policeman 88 55: Oh, yes indeed, mahster, Ah his sure ’nough wife.
1926		Van Vechten Nigger Heaven 14: Toly, you’s mah sho’ ’nough daddy an’ Ah sho’ does love you wid all mah h’aht.
1929		E. Booth Stealing Through Life in Hamilton Men of the Und. 262: I’ve lived in a sure-nuff hell down there.
1935		Z.N. Hurston Mules and Men (1995) 72: Lemme tell y’all a sho nuff tale ’bout Ole Massa.
1945		C. Himes ‘All He Needs is Feet’ in Coll. Stories (1990) 348: Now there’s that Mr Spencer, actin’ like a sho-nuff soldier.
1965		D. Pearce Cool Hand Luke (1967) 93: She knows a sure ’nough he-man when she sees one.
1969		‘Iceberg Slim’ Mama Black Widow 76: Ken yu change ’roun an bles me wif uh sho nuff job.
1974		V.E. Smith Jones Men 9: You letting his henchmen know you come in there for sho nuff business.
1979		‘Iceberg Slim’ Airtight Willie and Me 38: She [...] hailed a barmaid like she was setting in for some sho ’nuff tippling.
1981		H. Gould Fort Apache, The Bronx 45: José was hooked in with Hernando, who was the ‘sho nuff’ heavy dealer in the neighbourhood.
1990		S. Morgan Homeboy 225: Yous a binder Pie . . . eight / A shonuff grinder.
1999		‘Master Pimp’ Pimp’s Rap 34: Well Kid, you gotta sho’sho nuff burning desire to wanna be a pimp.