Wiktionary
plump down
vb. (context intransitive English) To sit down heavily onto a seat.
WordNet
plump down
v. drop heavily [syn: plonk down, plank down]
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]