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]