Half cocked

To go off half-cocked is to initiate a course of action when not fully prepared and not conversant with all the relevant facts. This figurative use of the expression dates from the early 19th century but its original, literal sense of course derives from the early days of firearms, from around the mid-1500s. The cocking mechanism on these old firearms was very ornate and resembled the head of a rooster, which is why it was called a cock; hence, the expression to cock a firearm dates from those early times. The cocking mechanism in these early firearms had two positions: fully cocked when the gun was ready to be fired and half-cocked when the cocking mechanism was drawn halfway back. A spring and a tumbler kept the gun in the half-cocked position so that it could be loaded. When in this half-cocked mode, the weapon could not be fired even when pulling on the trigger, so it represented an early form of safety catch. But, if the weapon was juggled about or dropped while in this mode, it could and frequently did discharge accidentally. The firearm was then said to have gone off ‘half-cocked’, which eventually came to mean an unprepared event. See also Flash in the pan.