Butt Flattener

—A long movie, usually around two and half hours without an intermission. Less commonly used to describe any media, such as books, that keep you sitting for a long period of time.
-May also refer to a body-shaping diet/article of clothing.

“ I’ve seen parts of The Shawshank Redemption. It’s SOOO long.”
“Two hours 20 minutes. It’s a bit of a butt flattener.”