Green’s Dictionary of Slang

anything considered hilariously funny.

1961		M. Braly Felony Tank (1962) 48: Agnes grinned at Billy. ‘Isn’t that a crack-up? I bet he gave those two clowns a fit.’.
1972		Eble Campus Sl. Oct. 2: crack up – something very funny or unusual.
1983		C. Heath A-Team 2 (1984) 161: The old man was a crack-up, too!
1997		C. Newland Scholar 100: You think I’m out of order, do you? Hear what, you know what I think? I think dat’s a fuckin’ crack up.
2000		J.J. Connolly Layer Cake 93: This gaff’ll interest ya [...] It’s a crack-up.