Emily Dickinson Lexicon

MOAT, n. [Ir. mota; Sp. id.; Fr. motte. The word signifies a bank or mound, that is, a mass or collection. This sense is transferred to the ditch adjoining, as dike is transferred to the bank.]

In fortification, a ditch or deep trench round the rampart of a castle or other fortified place. It is sometimes filled with water. – Encyc.