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.]

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.]

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.]

WordNet (r) 3.0 (2006):

plunk
    adv 1: with a short hollow thud; "plop came the ball down to the
           corner of the green" [syn: plop, plunk]
    n 1: a hollow twanging sound
    2: (baseball) hitting a baseball so that it drops suddenly [syn:
       plunk, plunker]
    v 1: make or move along with a sound as of a horse's hooves
         striking the ground [syn: clop, clump, clunk,
         plunk]
    2: set (something or oneself) down with or as if with a noise;
       "He planked the money on the table"; "He planked himself into
       the sofa" [syn: plank, flump, plonk, plop, plunk,
       plump down, plunk down, plump]
    3: drop steeply; "the stock market plunged" [syn: dive,
       plunge, plunk]
    4: pull lightly but sharply with a plucking motion; "he plucked
       the strings of his mandolin" [syn: pluck, plunk, pick]

Moby Thesaurus II by Grady Ward, 1.0:

156 Moby Thesaurus words for "plunk":
   abruptly, bang, bash, bat, beating, belt, biff, blow, bonk, bump,
   cast, chink, chop, chuck, clap, click, clink, clip, clobber, clop,
   clout, clump, clunk, coldcock, crack, crump, cut, dash, deal,
   deal a blow, deck, dig, dint, dive, drop, drub, drubbing, drumming,
   dull thud, exactly, fall, fetch, fetch a blow, flick, fling, flump,
   flush, full, fusillade, hastily, hit, hit a clip, hurl,
   impetuously, impulsively, jab, kerplunk, knock, knock cold,
   knock down, knock out, let have it, lick, like a flash,
   like a thunderbolt, nose-dive, of a sudden, on short notice, pad,
   parachute, paste, pat, patter, pelt, pick, pitapat, pitch,
   pitter-patter, plank, plop, pluck, plumb, plummet, plump, plunge,
   point-blank, poke, pop, pounce, pounce on, pounce upon, pound,
   precipitantly, precipitately, precipitously, precisely, punch, rap,
   right, sharp, skin-dive, sky-dive, slam, slap, slog, slug, smack,
   smack-dab, smash, smite, snap, soak, sock, sound, spang, square,
   squarely, startlingly, stoop, straight, strike, strike at, stroke,
   strum, sudden, suddenly, surprisingly, swat, sweep the strings,
   swing, swipe, swoop, swoop down, take a header, tap, tattoo, throw,
   thrum, thrust, thud, thump, thwack, tick, tinkle, toss, tunk,
   twang, unawares, unexpectedly, wallop, whack, wham, whop,
   without notice, without warning, yerk