The story
From groom (“officer of the English royal household”) + of + the + (close-)stool (“a chamber pot enclosed in a stool or box”).
Etymology from Wiktionary, CC BY-SA 4.0.
groom of the stool
Today it means: Originally an official responsible for helping the English monarch use the toilet; later a senior official who was allowed access to the monarch's privy chamber and served as a personal secretary. Full definition →