means A rounded heap or raised mass of earth, stones, or other material — natural or piled up — rising above the surrounding ground.

from From an Old English word 'mund,' meaning hand, protection, or guardianship — the same root behind ideas of defending and enclosing. The sense slid from 'protective boundary' to 'raised earthwork used as a defense' and finally to any heap or rise, possibly nudged along by Dutch and Low German cousins meaning hedge or embankment. The word's older guardian-self still lingers faintly: a mound was first something that kept things in, or kept enemies out.