“In stitches” means laughing very hard, often so hard that your sides hurt. It is a lively idiom for a moment when something is extremely funny, not just mildly amusing.
The image behind it is physical. A stitch is a sharp pain people sometimes feel in the side during exercise or heavy breathing. Strong laughter can create a similar tight, painful feeling in the stomach or ribs, so the expression suggests laughter so intense that it almost hurts.
Today, people use it in casual English to describe a very funny joke, story, show, or comment.
- Her story had us in stitches all through dinner.
- The audience was in stitches by the end of the comedian’s set.
It usually appears after a form of be, such as was in stitches or had us in stitches. It is stronger than simply saying something was funny. If a joke made you smile, you probably would not use this idiom. If you could barely stop laughing, it fits well.