belt up

verb (adverb)

slang to become or cause to become silent; stop talking: often used in the imperative
to fasten with or by a belt, esp a seat belt

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012