To be in stitches means that the individual finds himself or herself in a state of uncontrollable laughter, sometimes to the point of being in physical pain.
“Stitches” refers to a stitch in the side — a piercing sensation just below the ribcage and the best way to relieve the pain is to stop what you are doing and to press your hand just below the pain. As this gesture has also been associated with full bodied laughter over the centuries, having a side stitch and being in stitches referred to the pain experienced from overexertion of the torso.
The phrase was first used by Shakespeare in his play of 1602, Twelfth Night, where Maria says:
“If you desire the spleen, and will laugh yourself into stitches, follow me. Yond gull Malvolio is turned heathen, a very renegado; for there is no Christian, that means to be saved by believing rightly, can ever believe such impossible passages of grossness. He’s in yellow stockings. “
Despite appearing in a play by William Shakespeare, the phrase did not catch on as other phrases coined by Shakespeare caught on. Several generations later, in July 1914, The Lowell Sun reported that the community could count among its community ” … Ben Loring, a natural-born comedian, who seems to have no difficulty whatever in keeping his audience in stitches of laughter and glee.”
Historically Speaking 