The idiom "in stitches" originated from the world of medicine. In the past, "stitches" referred to the sutures used to close wounds or surgical incisions. When a person was said to be "in stitches," it meant they were laughing so hard that it felt as if they had burst their stitches or reopened a wound. Over time, the expression evolved to signify uncontrollable and hearty laughter, emphasizing the idea that something is incredibly funny.