moat: [14] The word moat originally meant a ‘mound’ or ‘embankment’ (this has since been hived off into the specialized form motte). The word was borrowed from Old French mote or motte ‘hill, mound’, whose ultimate source was probably a Gaulish mutt or mutta. The use of the word for the mound on which a castle keep was built led in Old French or Anglo-Norman to its reapplication to the ditch surrounding such a mound. [moat etymology, moat origin, 英语词源]