Green’s Dictionary of Slang

thigh-slapper n.

[the exaggerated slapping of one’s thighs to intimate the intensity of one’s hilarity]

(often used ironically) a supposedly very amusing joke.

[US]Wall Street Journal 13 Sept. 14/4: The thigh-slapper [...] the President got off to reporters when Lynda Bird showed up in a billowy muu-muu dress [OED].
[UK]Indep. Rev. 24 July 8: It will no doubt be a thigh-slapper for listeners of all ages.