Entering a situation without being fully prepared.
Origin: Muskets and revolvers have a lowered, half-cocked, and full-cock position for the hammer. During loading, the weapon is brought to a half-cock. The weapon must then be brought to a full-cock to be used. Forgetting to cock the weapon leads to it being unprepared for combat.
The boxer did'nt do any research on his opponent, and went off half-cocked to fight somebody twice his size.