Half-cock

[1][2] The still commonly used English expression of "going off half-cocked" derives from failing to complete the cocking action, leading to the weapon being unable to fire.

This is often used to describe someone acting prematurely, as in the case of one preparing to shoot their weapon without having set the firearm into "full-cock" position.

[3] Early flintlocks had required a half-cock position to prime the pan, in preparation for firing the firearm, after loading the main chamber.

[5] Nonetheless, some users did use the half-cock notch as a very early safety on such revolvers when all 6 chambers were loaded, often to a dangerous and unintended consequence.

Likewise, some early types of repeating lever action cartridge rifles—such as the Winchester Model 94 rifle—had a half-cock position that was intended to serve as a safety mechanism, to keep the hammer away from the firing pin while holding the rifle when a round was chambered, such as when in a blind awaiting game to appear.

Colt Single Action Army hammer at half cock
Cutaway view of the action of the Remington-Ryder No. 22 breech-loading rifle, which loads only with the hammer at half cock. Figure 1: Arm at the moment of discharge. Figure 2: Open to receive the cartridge. Figure 3: At half cock, loaded.