Green’s Dictionary of Slang

a certainty; also attrib.

1981		J. Wambaugh Glitter Dome (1982) 175: Talk about a slam dunk! [...] Baby, that was a motherfucking ex-plosion!
1999		Guardian G2 2 Aug. 5: We Britons draw the line at barbarisms like ‘slam-dunk’ (’sure thing’; basketball, don’t you know).
2006		G. Pelecanos Night Gardener 33: It’s almost a slam dunk. Let’s take it to the finish line.
2009		N. Flexner Disassembled Man [ebook] Even if we can’t get you for the others, this one’s a slam dunk. Witness, Fingerprints. Motive.
2012		L. Berney Whiplash River [ebook] Evelyn [...] helped build a slam-dunk case against the Armenian mob.
2017		D. Winslow The Force [ebook] No question about it [...] t’s a slam dunk.
2023		M. Herron Secret Hours 387: [S]he had him on tape with De Vries, discussing ways of milking the vetting protocols, so a conspiracy charge was a slam dunk.