means Rounded and full-bodied in a soft, fleshy way — comfortably padded rather than thin.

from From Middle Dutch and Middle Low German 'plomp,' meaning blunt, dull, or thick — a word that also carried the sense of clumsy and heavy. English borrowed it in the 1400s, and curiously its early meaning was closer to 'dull-witted' or 'rude.' The 'rounded and full' sense came later, likely shaded by the related sound-word 'plump' (the heavy thud of something dropping), which is itself imitative — the noise of weight landing. So the soft, well-fed meaning we use today grew out of an old word for thick and heavy.