1550s, "hedge, fence," also "an embankment, a dam" (a sense probably influenced by mount (n.)), a word of obscure origin. The relationship between the noun and the verb is uncertain.
Commonly supposed to be from Middle English mounde "the hand; guardianship, power," from Old English mund (cognate with Latin manus), but this is not certain (OED discounts it on grounds of sense). Perhaps it is a confusion of the native word and Middle Dutch mond "protection," used in military sense for fortifications of various types, including earthworks.
From 1726 as "artificial elevation of earth" (as over a grave); 1810 as "natural low elevation." As the place where the pitcher stands on a baseball field, from 1912. Mound-builder "one of the prehistoric race of the Mississippi Valley that erected extensive earthworks" is by 1838.
In Middle English mounde also meant "the world," from Old French monde, from Latin mundus (see mundane).
1510s, "to enclose with a fence;" c. 1600 as "to enclose or fortify with an embankment;" see mound (n.). From 1859 as "to heap up." Related: Mounded; mounding.
c. 1300, mote "a mound, a hill" (a sense now obsolete); late 14c., "ditch or deep trench dug round the rampart of a castle or other fortified place," from Old French mote "mound, hillock, embankment; castle built on a hill" (12c.; Modern French motte) and directly from Medieval Latin mota "mound, fortified height," a word of unknown origin, perhaps from Gaulish mutt, mutta.
The sense shifted in Norman French from the castle mound to the ditch dug around it. For a similar evolution, compare ditch (n.) and dike. As a verb, "to surround with a moat," early 15c. Related: Moated.