Buckle up

Buckle up as a phrase dates from the early 1500s, and until swords were generally discarded as weapons of war during the 19th century, it meant 'prepare for battle', when one would invariably buckle a sword belt to one's waist. From this original meaning, its figurative usage, meaning to get ready and prepare for anything, was in full swing by the late 1500s, and is sometimes still used in this way today. Since the introduction of car seat belts in the second half of the 20th century, buckle up is commonly used as an injunction to fasten one's seat belt in a car, although it was also used in airplanes before this. See also Belt up.