To be “in stitches” means to laugh uproariously or uncontrollably. The saying has been used in English for hundreds of years and even appears in a Shakespeare play. It refers to a common physical ailment, a side stitch, which occurs during periods of high physical exertion. It is possible to experience a side stitch from laughter, leading to expressions such as “in stitches from laughing so hard.” Most people described this way are not actually experiencing side stitches from laughter; this is an example of creative exaggeration, or hyperbole.
The word “stitch” comes from the Old English root word “stice,” meaning to stab or puncture. "Stice” provides the modern words “stick” and “stitch,” which can both mean to puncture something with a sharp object. The side stitch was so named because the sensation is a stabbing pain. The cause of side stitches is a matter of medical conjecture. It is generally believed to be associated with stress to the diaphragm, the abdominal muscle that controls lung activity.
As laughter involves the lungs and the diaphragm, it is possible to get side stitches from prolonged laughter. Laughter is often a social phenomenon; people tend to laugh more when other people are laughing or if they have been laughing previously. Thus, a particularly funny story or performance can cause people to laugh so much that they actually experience physical discomfort, such as shortness of breath or side stitches. Originally, this was expressed with sayings like “throw him into stitches.” Eventually it was shortened to “in stitches,” the side stitch being a common enough experience that no explanation was necessary.
The first recorded use of the expression “in stitches” occurs in William Shakespeare’s play Twelfth Night, written around 1601. After executing a practical joke, the character Maria tells her co-conspirators, “If you will laugh yourself into stitches, follow me.” Shakespeare’s casual usage suggests the phrase had already been in conversational use for some time. The actual date it was first used is unknown. It is one of many expressions used by Shakespeare that holds the same meaning in the present day.
Writers and critics will often say that a comedic performance or movie kept the audience in stitches. Of course, this does not mean that audience members were literally experiencing physical discomfort. This use of the phrase, probably the most common modern usage of “in stitches,” is an example of hyperbole. Hyperbole is the deliberate use of exaggeration for dramatic or comic effect.