I learned the origin of the term "going off half-cocked" recently. On muskets, you half-cock to put the gun in a position of safety. You can't pull the trigger when it's half-cocked (you literally can't, it won't move, unless of course the gun is malfunctioning). Half-cock is a safe position to handle the weapon, march with it, load powder into the pan, etc. When you go off half-cocked it means you're ill-prepared, and a little stupid.