Your ass sucks buttermilk

"Your ass sucks buttermilk" is a phrase said in events of disbelief. Used in the same context as "bullshit!" or "no way!" when someone hears something incredible or unbelievable.

Ray: Hey Rog, guess what happened this morning at the coffee shop.
Roger: What?
Ray: Apparently John's wife found out he was cheating on her ass, and drove her car through the wall this morning while we were sitting in there.
Roger: Your ass sucks buttermilk!