Be in charge, boss others, as in In our division the chairman's son rules the roost. This expression originated in the 15th century as rule the roast, which was either a corruption of rooster or alluded to the person who was in charge of the roast and thus ran the kitchen. In the barnyard a rooster decides which hen should roost near him. Both interpretations persisted for 200 years. Thomas Heywood (c. 1630) put it as “Her that ruled the roast in the kitchen,” but Shakespeare had it in 2 Henry VI (1:1): “The new-made duke that rules the roast,” which is more ambiguous. In the mid-1700s roost began to compete with roast, and in the 1900s roost displaced roast altogether. Also see run the show.

To dominate; to be in charge: “Even though Sally has five older brothers, she still rules the roost.”