Batty flap 

Batty means bum and flap usually means the cheeks. Its an urban British slang and for some strange reason others have gotten a different meaning of it. Like how the word faggot which is actually a name for a type of food some how got converted into a nasty insult towards homosexuals by people. Usually it is just used because its funny and more polite than saying arse cheeks.

I will shove that up your batty in a minute. Explanation: I will shove that up your ass - Used when somebody is annoying you with something.
My batty flaps are flapping because I just farted. Explanation: My bum cheeks are shaking from my farting.

Batty flap by Flapping Batty January 24, 2016

Batty flap 

The flap that surrounds the female genitals,also used as an insult to a minor orlesser being

I corressed my female friends batty flap, thy friend of mine

Batty flap by Chris Burgess November 15, 2004