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1.
The bottom cushion in a car seat where yor ass rests.

As opposed to the companion"back cushion"
"When my girl crashed my car she shit herself which ruined the ass cushion in my caddy."
by Mike Cheezmar December 06, 2012
 
2.
A pillow normally used to sleep on that is used under ones behind to make sitting more comfortable.
Why is my pillow being used as an Ass Cushion?
by Ms. Wrong Generation August 03, 2010