From The Collaborative International Dictionary of English v.0.48:
Plunk \Plunk\, v. t. [Imitative.] [Chiefly Colloq.]
1. To pluck and release quickly (a musical string); to twang.
[Webster 1913 Suppl.]
2. To throw, push, drive heavily, plumply, or suddenly; as,
to plunk down a dollar; also, to hit or strike.
[Webster 1913 Suppl.]
3. To be a truant from (school). [Scot.]
[Webster 1913 Suppl.]
From The Collaborative International Dictionary of English v.0.48:
Plunk \Plunk\, v. i. [Chiefly Colloq.]
1. To make a quick, hollow, metallic, or harsh sound, as by
pulling hard on a taut string and quickly releasing it; of
a raven, to croak.
[Webster 1913 Suppl.]
2. To drop or sink down suddenly or heavily; to plump.
[Webster 1913 Suppl.]
3. To play truant, or "hooky". [Scot.]
[Webster 1913 Suppl.]
From The Collaborative International Dictionary of English v.0.48:
Plunk \Plunk\, n.
1. Act or sound of plunking. [Colloq.]
[Webster 1913 Suppl.]
2. [Slang]
(a) A large sum of money. [Obs.]
(b) A dollar. [U. S.]
[Webster 1913 Suppl.]