An Etymological Dictionary of the German Language/Annotated/plump

by Friedrich Kluge

plump, adjective, ‘plump, unwieldy, coarse,’ Modern High German only, from Low German and Dutch plomp, ‘thick, coarse, blunt’ hence in Swiss pflumpfig, with the High German permutation); from Dutch the word seems to have passed into English and Scandinavian as plump. The term plump was originally an imitation of sound.