What is the etymology of sidesplitting? In particular, is there a physical interpretation of sides splitting?
Old English side “flanks of a person, the long part or aspect of anything”. . . . Side-splitting “affecting with compulsive laughter” is attested by 1825.
The simple physical interpretation is that “side-splitting” laughter forces convulsions so powerful that they could split your sides wide open. It’s clearly hyperbole, but the feeling is easily imagined by anyone who has experienced a real side-splitter.