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1550s, "hedge, fence," also "embankment, dam" (a sense probably influenced by mount (n.)). The relationship between the noun and the verb is uncertain. Commonly supposed to be from Old English mund "hand, protection, guardianship" (cognate with Latin manus), but this is not certain (OED discounts it on grounds of sense). Perhaps 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" (as over a grave); 1810 as "natural low elevation." As the place where the pitcher stands on a baseball field, from 1912.
1510s, "to enclose with a fence;" c.1600 as "to enclose with an embankment;" see mound (n.). From 1859 as "to heap up." Related: Mounded; mounding.