The Collaborative International Dictionary of English v.0.48:

Plump \Plump\ (pl[u^]mp), a. [Compar. Plumper
   (pl[u^]mp"[~e]r); superl. Plumpest.] [OE. plomp rude,
   clumsy; akin to D. plomp, G., Dan., & Sw. plump; probably of
   imitative origin. Cf. Plump, adv.]
   1. Well rounded or filled out; full; fleshy; fat; as, a plump
      baby; plump cheeks. --Shak.
      [1913 Webster]

            The god of wine did his plump clusters bring. --T.
                                                  Carew.
      [1913 Webster]

   2. Done or made plump, or suddenly and without reservation;
      blunt; unreserved; direct; downright.

            After the plump statement that the author was at
            Erceldoune and spake with Thomas.     --Saintsbury.
      [Webster 1913 Suppl.]