The commode or chamberpot was called a stool in the 1500s and earlier. Before running water, the commode was a box with a hole in it. The waste has to be removed from the room by a person.
Early English monarchs actually had a member of their retinue called the “Groom of the Stool” whose role was originally to help the king use the “stool” in as dignified a manner as possible. The title was highly sought after for the younger members of the aristocracy because of the access it afforded to the seat of power in the kingdom.
There were similar roles for other powerful families. It was typical for the private room of a nobleman to have second door for the exclusive use of this member of their household.
Anyway: the stool/commode eventually became associated with what it was used for.
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