If a person Rules The Roost they're in charge and demonstrate their authority. The phrase has been used since the 1500s and was then popularised by Shakespeare in Henry VI Part II: "Suffolk, that new made man that rules the roast." In fact, tradition has it that the master of the house carves and serves the roast meat. In 1637 Thomas Nabbes wrote in "I am my lady's cook, and king of the kitchen where I rule the roast." So, it looks like the phrase originally to do with chickens (except roast ones). This is further supported by the fact that in Anglo-Saxon English the word roast was pronounced with a long "o", making it sound like roost so the phrase used to be "ruling the roast".