in stitches

It means that the person is laughing uproariously. When that happens, you might laugh so hard that your stomach hurts. A sudden cramp in the stomach or side is often called a "stitch," because it is like being stabbed--the earliest meaning of stitch was "to prick or to stab" with a sharp object. The meaning of sewing shoes or clothing came later.

To be"in stitches" doesn't necessarily mean that your stomach hurts, just that you can't stop laughing.

Take a look here In stitches for more information. I didn't know that the first recorded use comes from Shakespeare.

edited to add: A better explanation

Another stitch idiom is to "not have a stitch on," to be naked. "Stitch" also had an earlier meaning of "a piece of clothing."

Edited by: nutraxfornerves

Thank you for the explanation and the links, Nutrax!

You can also find yourself 'stitched-up'. Meaning someone has contrived to make sure that you take the blame for something. Crook often says (in a film), "They've stitched me up!"

You can also find yourself 'stitched-up'. Meaning someone has contrived to make sure that you take the blame for something. Crook often says (in a film), "They've stitched me up!"

Apparently it's UK slang. I've never heard of it. From The Cambridge Dictionaries online:

to intentionally make someone look guilty of doing something which they did not do
He claims he was stitched up by the police.

There's always "a stitch in time saves nine." Use one stitch to sew up a small hole in your clothes now, or delay and have to use nine stitches because the hole will get bigger. In other words, deal with some problem now, before it gets worse. Sometimes shortened to just "a stitch in time." Dates to early 18th C.

There's always "a stitch in time saves nine." Noted, Nutrax.

Stitch is still sometimes used for clothing, "running around without a stitch on" i e naked.

cf. To have someone in stitches - Glaswegian for violence.