To allow one's posture to slump or droop downward. I could see the kids starting to slouch down with boredom, so I knew it was time to start wrapping up the trip to the museum. Please stop slouching down in your chair like that—it's not good for your back!
slouch down (in something)
to sink or snuggle down into something, trying to become less visible or more comfortable. Please don't slouch down in your chair, Tim. He can't sit in anything without slouching down.
to slump or droop down. Don't always slouch down, Timmy! Stand up straight. I slouch down because I am tired.