pan cover; touch-hole cover
Historical term for the metal cover over the priming pan of a matchlock or cannon. Now mostly encountered in the idiom 火蓋を切る (to start a battle or contest).
See also: 火蓋を切る
Open the pan cover of the matchlock and place the priming powder.
The expression 火蓋を切る is used to mean 'to open hostilities' or 'to start a battle'.