butt coaster

A towel, or other absorbent material, used to protect a bed, bench, chair, etc., from the residue moisture left over on one's posterior after a shower, bath, or swim. The towel acts much like a coaster that is used to protect furniture, except this time it's protecting the bed from your wet butt.

"Can't you please use a butt coaster" said Jean, after I got our bed wet by sitting directly on it after coming out of the shower.