This is a phrase used to refer to someone who causes problems. This is said to people who are trouble makers, people who are always stirring up trouble. The phrase comes from the way a baby cries when their cradle is rocked, hence the term.
Hey man, you better not go into that door, or else you're gonna rock the cradle.
N. This term describes the one high school clique in which all of it's associates are physically attractive. They are usually known to be snobby, and will not talk to anyone besides themselves. They will usually grow up to be unsuccessful.