split one's sides - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, laugh one's head off. Be extremely amused, laugh uproariously. For example, That comedian had us splitting our sides, or Jane laughed her head off when she saw Rob's costume. The first of these hyperbolic terms dates from about 1700.