What does "plumper" mean?

Plumper: (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.

Additional senses

  1. 2.A voter who plumps his vote. [Eng.]
  2. 3.A downright, unqualified lie. [Colloq. or Low]

Sources