jump the gun 

to do something too soon, especially without thinking carefully about it (Originally used in sports contests that are started by firing a gun. If someone running in a race jumps the gun, they start running before the gun has been fired to start the race.)

He shouted at me before I had time to explain, but later he apologised for jumping the gun (jump the gun).

jump the gun by nemez January 16, 2011

jump the gun 

to do something prematurely

I think I jumped the gun on calling her "bitch." she won't talk to me anymore, I guess it was too early.

jump the gun by bitch November 21, 2002

jump the gun 

To commit suicide, not necessarily by use of a gun.

Bob Jumped the gun because his wife was cheating on him.

jump the gun by some guy November 21, 2002