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1.
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.
by bitch November 21, 2002
 
2.
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).
by nemez January 14, 2011
 
3.
To commit suicide, not necessarily by use of a gun.
Bob Jumped the gun because his wife was cheating on him.
by some guy November 21, 2002