look up any word, like cleveland steamer:
 
1.
When a young male lowers his standards for casual sexual encounters. "Slut humping."
"I feel like there are no interesting girls here.... I think I'm going to end up slumping again for my birthday."
by ogmarshall January 13, 2009
 
2.
One Human Humping Another while Asleep...

Sleep + Humping.... often done when both partners have PJs on.
One Night after a Long day at Work Bob Briefly Slumped Jane.... the Slumping was not of a duration that Jane found Acceptable.

Jane was left unfullflled.
by DanielD June 14, 2010
 
3.
Having sex or romping in a sleeping bag.
Me and jon saw two people slumping while we were walking around the campsite.
by Mrp3anut117 August 21, 2008
 
4.
Taking a dump while sleeping.

Hence Slumping.
#1: Bro, you were in the can for 2 hours, are you alright?

#2: I was slumping, ya prick.
by DOOP DOOP DA DA DA May 11, 2010