ass cushion 

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."

ass cushion by Mike Cheezmar December 8, 2012

Ass Cushion 

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?

Ass Cushion by Ms. Wrong Generation August 3, 2010