cheek flapper 

To expell gas from one's anus with such force that both sides of his/her buttox slap together to create a distinctly audible, and occasionally painful, vibration.

During the awkward silence on his date, John let forth such an intense cheek flapper that the silverware on the table rattled.

cheek flapper by Shpoiken April 28, 2005

Flapper cheeks

But cheeks that are so saggy that they can make that person fly.

I once saw some damn flapper cheeks!

Flapper cheeks by Amelia.Summers October 17, 2020