1. To make plump; to fill (out) or support; — often with up.plump up
the pillows
To plump up the hollowness of their history with improbable miracles.
Fuller.

2. To cast or let drop all at once, suddenly and heavily; as, to plump a stone into water.

3. To give (a vote), as a plumper. See Plumper, 2.

PLUMP
Plump, adv. Etym: [Cf. D. plomp, interj., G. plump, plumps. Cf.
Plump, a. &v.]

Defn: Directly; suddenly; perpendicularly. "Fall plump." Beau. & Fl.

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.

2. (English Elections)

Defn: 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.

3. A voter who plumps his vote. [Eng.]