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).
stool (stōōl)
n.
A discharging of the bowels.
Evacuated fecal matter.
(also stoolie) A police informer; stool pigeon: He's nothing but a cop's stool (Underworld 1906+, variant 1924+)
verb: to make me stool on a friend (1911+)
[back formation fr stool pigeon]