E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.