plumpness (n.)

late 15c., "blunt, dull" (in manners), from Dutch plomp "blunt, thick, massive, stumpy," probably related to plompen "fall or drop heavily" (see plump (v.)). Meaning "full and well-rounded," of a person, "fleshy, chubby," is from 1540s in English. Danish and Swedish plump "rude, coarse, clumsy" are from the Low German word and represent a different sense development. Middle English plump (n.) "a compact group of people, a crowd" (c. 1400) perhaps is from Middle Dutch as well.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).