means A deep, wide ditch—usually filled with water—dug around a castle or fortified building to keep attackers out.

from From Old French 'mote,' which oddly meant a mound or embankment of earth, not the ditch itself. The sense flipped over time: dig a defensive trench and you pile up the dug-out dirt as a hill, so 'mote' came to mean the watery hollow rather than the heap beside it. English borrowed it in the medieval period, eventually settling on the spelling 'moat.'