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split one's sides

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.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.