Green’s Dictionary of Slang

extremely funny; also as n.; thus side-splittingly adv.

1860		S. Mordecai Virginia in By-gone Days 188: These among other side-splitting tales, which he told and acted with the skill of a Matthews.
1870		Dublin Wkly Nation 29 Oct. 1/2: The side-splitting anecdotes of Dean Swift and his Witty man.
1881		Daily Tel. 27 Dec. n.p.: This [...] past master of the art of side-splitting [OED].
1890		J. Payn Notes from ‘News’ 166: Of course, ‘Greek Wit’ is not always of this side-splitting description.
1900		Irish Indep. (Dublin) 26 Oct. 4/1: Read the Side-Splitting Adventures of Darby Durken, P.L.G.
1912		A. Bennett ‘The Lions Share’ in Matador of Five Towns 244: In the Five Towns the following history is [...] something side-splittingly funny—one of the best jokes that ever occurred.
1926		Hull Dly Mail 20 May 3/4: Side-splitting [...] ‘My dear, I laughed so hard that [etc]’.
1930		Era (London) 5 Mar. 9/2: A jolly, preposterous, side-splitting story.
1947		Birmingham Dly Gaz. 15 July 2/7: A side-splitting farce.
1954		K. Amis letter 23 Jan. in Leader (2000) 363: ‘Kingsley Amis’s uproarious, side-splitting, tear-jerking, cock-standing’ etc.
1970		Daily Tel. 6 Aug. 6/2: The reader is [...] given an at times side-splittingly funny account of the eccentricities of Frazer and his wife [OED].
1999		Guardian Guide 4–10 Sept. 18: Literary references that punters with an A-level knowledge of the classics found side-splitting.
2016		Springfield News-Leader (MO) 23 June W9/1: Nonstop plot twists, side-splitting dialogue [...] add to the zany fun.