1481, "blunt, dull" (in manners), from Du. plomp "blunt, thick, massive, stumpy," probably related to plompen "fall or drop heavily" (see plump (v.)). Meaning "fleshy, of rounded form" is from 1545.
c.1300, "to fall or strike with a full impact," from M.Du. plompen, or M.L.G. plumpen, probably of imitative origin. Meaning "to plump (something) up, to cause to swell" is first recorded 1533, from the adj.