An Etymological Dictionary of the German Language/plump

by Friedrich Kluge

plump, adj., ‘plump, unwieldy, coarse,’ ModHG. only, from LG. and Du. plomp, ‘thick, coarse, blunt’ hence in Swiss pflumpfig, with the HG. permutation); from Du. the word seems to have passed into E. and Scand. as plump. The term plump was orig. an imitation of sound.