Half-cock

Half-cock is when the position of the hammer of a firearm is partially—but not completely—cocked. Many firearms, particularly older firearms, had a notch cut into the hammer allowing half-cock, as this position would neither allow the gun to fire nor permit the hammer-mounted firing pin to rest on a live percussion cap or cartridge. The purpose of the half-cock position has variously been used either for loading a firearm, as a safety mechanism, or for both reasons. The still commonly used English expression of "going off half-cocked" derives from the failure of the half-cock mechanism to keep the hammer from falling and the subsequent failure to prevent a firearm from firing when unintended.