Green’s Dictionary of Slang

something or someone exceedingly funny.

1830		Sorrow-disperser, Or Humpy Funnydoss' Bundle of Mirth [contributor] Jacob Sidesplitter.
1854		Household Words 23 379/1: Professing an intent to be a side-splitter of no ordinary width of aperture, it [i.e. a play] was conducive rather to a pensive frame of mind.
1854		Nashville Union & American (TN) 18 May 2/3: IThe following ‘side splitter’ [...] should be read by every one afflicted with the ‘blues’.
1864		Harper’s Mag. Feb. 422/1: I send you three samples [of letters] [...], hoping thereby to reciprocate some of your side-splitters .
1866		S.W. Pierce Battlefields & Campfires of 38th 91: The manner of the man and the joke was a side-splitter, and we laughed heartily.
1872		Surry Comet 4 May 2/3: A side-splitter quite beyond a joke.
1884		R.C. Hartranft [title] Journal of Solomon Sidesplitter.
1890		Sporting Times 22 Feb. 3/1: In this age of antediluvian stories and B.C. side-splitters, it is quite refreshing to fall across a joke now and then in its swaddling clothes.
1903		A. Binstead Pitcher in Paradise 133: As regards poetry I have already had a sidesplitter entitled ‘Don’t chalk your cue before a lady’ accepted by Mr Arthur Robert.
1916		Shields Dly Gaz. 17 May 1/2: Syd Chaplin In the Latest Keystone Farce [...] A Real Side-splitter.
1925		Birmingham Dly Gaz. 31 Mar. 3/4: A home circus that is a marvel as a side-splitter.
1936		Coventry Eve. Teleg. 10 Mar. 4/2: Stanley Lupino is a real side-splitter.
1950		Sketch (London) 22 Nov. 36/3: What a scream it will be! What a side-splitter!
1961		Stage (London) 23 Feb. 7/1: His humour [...] was patchy — one minute he’d come up with a side-splitter, then he’d inflict a couple of egg-layers.
1987		Stage (London) 29 Oct. 7/3: Her great [...] ability to take even the most mundane event and [...] turn it into a side-splitter.
1999		Guardian G2 27 July 23: No side-splitter, but fluid direction and upper-crust cast.