Belt up

British slang for shut up which dates from the 1930s. Eric Partridge's Dictionary of Slang maintains the source is RAF slang, but gives no further explanation. No one is certain how belt up came to mean shut up. Perhaps the allusion is to wearing a belt around one's mouth, but this is pure guesswork. Some sources maintain the origin is from WWI when belt up was a common injunction to arm machine guns with belts of ammunition, but how this progressed to mean shut up remains obscure and unproven. Belt up, like buckle up, is also an injunction to wear car seat belts, which became a federal law in the US in 1968, but is generally left to individual states to interpret and/or enforce. The wearing of car seat belts became compulsory in Britain in 1983. See also buckle up