stoolyoudaoicibaDictYouDict[stool 词源字典]
stool: [OE] Although stools are for sitting on, the word’s etymological meaning is ‘stand’. It comes from a prehistoric Germanic *stōlaz, which was formed from the base *stō-, *sta- ‘stand’ (source of English stand) using the noun suffix *-l- (in much the same way as saddle was formed from a base meaning ‘sit’). The notion of ‘standing’ no doubt passed into ‘sitting’ via an intermediate generalized ‘be positioned or situated’.

In the 15th century stool came to be applied specifically to a ‘commode’, and this led to its use in the following century for an ‘act of defecating’, and hence for a ‘piece of faeces’. Stoolpigeon [19] originated in American English as a term for a decoy pigeon tied to a stool.
=> stall, stand[stool etymology, stool origin, 英语词源]
stool (n.)youdaoicibaDictYouDict
Old English stol "seat for one person," from Proto-Germanic *stolaz (cognates: 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" (cognates: Lithuanian pa-stolas "stand," Old Church Slavonic stolu "stool;" see stet).

Originally used of thrones (as in cynestol "royal seat, throne"); decline in sense began with adoption of chair (n.) from French, which relegated stool to small seats without arms or backs, then to "privy" (early 15c.) and thence to "bowel movement" (1530s).