flump vs plump oneself down

flump

noun
  • A type of large marshmallow. 

  • A fat out-of-shape person. 

  • The dull sound so produced. 

verb
  • To move or fall heavily, or with a dull sound. 

  • To drop something heavily or with a dull sound. 

plump oneself down

How often have the words flump and plump oneself down occurred in a corpus of books? (source: Google Ngram Viewer )