Old English stol "seat for one person," from Proto-Germanic *stolaz (cf. Old Frisian stol, Old Norse stoll, Old High German stuol, German Stuhl "seat," Gothic stols "high seat, throne"), from PIE *sta-lo-, locative of root *sta- "to stand" (cf. Lithuanian pa-stolas "stand," Old Church Slavonic stolu "stool;" see stet).
Originally used of thrones (cf. cynestol "royal seat, throne"); change of meaning began with adoption of chair from French, which relegated stool to small seats without arms or backs, then "privy" (early 15c.) and thence to "bowel movement" (1530s).