Butt Flapping 

When you shake your butt in a way to make the cheeks sound like wings flapping

Wes was butt flapping so hard I thought it was a flock of geese

Butt Flapping by Marginallyclean July 11, 2016

Buttclapping

Unwittingly making a clapping sound with your butt cheeks while exercising.

My buttclapping was so loud while I did my rebounder workout!

Buttclapping by madlove10 March 9, 2017