BEAT THE GUN

{v. phr.} 1. To start before the starter's gun in a race. * /The runners were called back because one of them jumped the gun./ 2. {informal} To start before you should; start before anyone else. * /The new students were not supposed to come before noon, but one boy jumped the gun and came to school at eight in the morning./ * /The students planned to say happy birthday to the principal when the teacher raised her hand, but Sarah jumped the gun and said it when he came into the room./