The old flintlock muskets, rifles and fowling pieces (shotguns) were built in such a way as to require the shooter to pull the hammer back before it could be loaded. The hammer was in the way of the frizzin. A weapon in good condition could not be fired if it was half cocked. As they wore out they sometimes went off half cocked.
Cocked simply means the hammer is back and ready to fire. You would not want to go around with your hammer way back unless you wanted a sex change and couldn't afford a doctor.