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.

Got the covid-19 vaccine

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