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.
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   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.]

The Collaborative International Dictionary of English v.0.48:

Plumper \Plump"er\, n.
   1. One who, or that which, plumps or swells out something
      else; hence, something carried in the mouth to distend the
      cheeks.
      [1913 Webster]

   2. (English Elections) A vote given to one candidate only,
      when two or more are to be elected, thus giving him the
      advantage over the others. A person who gives his vote
      thus is said to plump, or to plump his vote.
      [1913 Webster]

   3. A voter who plumps his vote. [Eng.]
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   4. A downright, unqualified lie. [Colloq. or Low]
      [1913 Webster]