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 (n.)
mid-14c., from Old French mote "mound, hillock, embankment; castle built on a hill" (12c.; Modern French motte), from Medieval Latin mota "mound, fortified height," of unknown origin, perhaps from Gaulish mutt, mutta. Sense shifted in Norman French from the castle mound to the ditch dug around it. As a verb, "to surround with a moat," early 15c.