belt up

The phrase "belt up" is an idiomatic expression that means to fasten one's seatbelt in a vehicle. It is often used as a command or a request for someone to secure their seatbelt for safety purposes while driving or riding in a car, airplane, or any other type of transportation. "Belt up" can also informally mean to be quiet or stop talking, but in the context of transportation, it primarily refers to fastening a seatbelt.