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).