Butt-Smashed
Taking it in the butt hard.
I butt-smashed my girlfriend last night. She was walking funny for hours.
Smashing butt cheeks
The act of sex in which a man enters his lady friend from behind and as he thrusts he smashes her lovely butt cheeks.
Yo, Tommy Gun, Liz and I were smashing butt cheeks in my dorm room when my suite mate walked in!