jump the gun

[jump the gun] also [beat the gun] {v. phr.} 1. To start before thestarter's gun in a race.

The runners were called back because oneof them jumped the gun.

2. {informal} To start before you should;start before anyone else.

The new students were not supposed tocome before noon, but one boy jumped the gun and came to school ateight in the morning.

The students planned to say happy birthdayto the principal when the teacher raised her hand, but Sarah jumpedthe gun and said it when he came into the room.