Green’s Dictionary of Slang

(US) an urgent need to visit the lavatory; the inability to control one’s bowels and/or bladder.

1910		B.L. Bowen ‘Word-List From Western New York’ in DN III:vi 448: shorts, n. Diarrhea.
1995		R. Campbell Wizard of La-La Land (1999) 108: All he said was he’d had his gall bladder removed and when he ate anything greasy he got the shorts.