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Wiktionary
plump down

vb. (context intransitive English) To sit down heavily onto a seat.

WordNet
plump down
  1. v. drop heavily [syn: plonk down, plank down]

  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, plunk down, plump]