Green’s Dictionary of Slang

(often used ironically) a supposedly very amusing joke; cite 1957 suggests the physical emphasis.

1950		Press & Sun-Bulletin (Binghamton, NY) 17 Sept. 22/4: Here is a really funny book [...] Mr Taylor has produced a thigh-slapper this time.
1957		Birmingham Dly Post 3 Apr. 25/6: Dame Peggy [Ashcroft] could never be an arch young woman, a thigh-slapper. She is an actress whose gaiety is born of truth.
1965		Wall Street Jrnl 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].
1971		Province (Vancouver,) 6 Jan. 25/6: Somehow the French and the English didn’t see eye-to-eye, or thigh-slapper to thigh-slapper, when it came to merriment.
1978		Gettysburg Times (PA) 22 Dec. 29/3: Stars have some thigh-slappers. The stars have their favorite Christmas jokes.
1984		Tennessean (Nashville, TN) 5 Feb. 27/2: [headline] No Thigh-Slappers From the Democrats.
1999		Indep. Rev. 24 July 8: It will no doubt be a thigh-slapper for listeners of all ages.
2000		Sydney Morn. Herald6 May 38/2The Anthony ‘Choc’ Mundine saga has been a thigh-slapper for a fortnight: .
2010		Calgary Herald (Alberta) 6 May 67/1: That would be a thigh-slapper coming from anybody but one of [...] the Staal brothers.
2018		Manitoba Herald-Times (WI) 21 Sept. A1/1: The critics say it’s going to be a real thigh-slapper.