moat n.

"a trench made by digging," especially a trench for draining wet land," Middle English diche, from Old English dic "ditch, dike," a variant of dike (q.v.), which at first meant "an excavation," but later in Middle English was applied to the ridge or bank of earth thrown up in excavating. Middle English diche also could mean "a defensive wall."

As the earth dug out of the ground in making a trench is heaped up on the side, the ditch and the bank are constructed by the same act, and it is not surprising that the two should have been confounded under a common name. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]

Ditch-water "stale or stagnant water that collects in ditches" is from mid-14c. In Middle English, digne as dich water (late 14c.) meant "foolishly proud." Also see last-ditch.

Old English dic "trench, ditch; an earthwork with a trench; moat, channel for water made by digging," from Proto-Germanic *dikaz (source also of Old Norse diki "ditch, fishpond," Old Frisian dik "dike, mound, dam," Middle Dutch dijc "mound, dam, pool," Dutch dijk "dam," German Deich "embankment"), from PIE root *dheigw- "to pierce; to fix, fasten." The sense evolution would be "to stick (a spade, etc.) in" the ground, thus, "to dig," thus "a hole or other product of digging."

This is the northern variant of the word that in the south of England yielded ditch (n.). At first "an excavation," later applied to the ridge or bank of earth thrown up in excavating a ditch or canal (late 15c.), a sense development paralleled by the cognate words in many languages, though naturally it occurred earlier in Dutch and Frisian. From 1630s specifically as "ridge or bank of earth to prevent lowlands from being flooded." In geology, "vertical fissure in rocks filled with later material which made its way in while molten" (1835).