slump down in
slump down in (something)
To droop, fall, or collapse physically downward into or while in something. A few students were slumping down in their chairs, very obviously trying to sleep. She was so exhausted when she came in from work that all she could do was slump down in the recliner.
slump down in(to) something
[for someone] to bend down or collapse into something, such as a chair or bed. Gary grabbed at his chest and slumped down into the bed. He slumped down into the chair and draped himself over the arm.