Brewer's: Letter of Marque
A commission authorising a privateer to make reprisals on a hostile nation till satisfaction for injury has been duly made. Here “marque” means march, or marca, a border-land (whence our “marquis,” the lords appointed to prevent border-incursions). A letter of marque or mart was permission given for reprisals after a border-incursion. Called jus marchium.