plump down

[Merna]

set (something or oneself) down with or as if with a noise

[Rihanna]

To plump down generally refers to the act of sitting or lying down heavily or suddenly due to tiredness or lass. It might also refer to paying or giving a large amount of money all at once or abruptly. The context of the phrase will dictate its precise meaning.

[Marquetta]

To sit down heavily onto a seat.