This is a very interesting question!
Although it depends on the context someone wrote, I would probably say 'お尻を出して馬鹿にする' or 'お尻を出して侮辱する.' It could just be me, though.
'お尻を出して馬鹿(ばか)にする' = to expose one's naked buttocks (in a public place) as a joke
'お尻を出して侮辱(ぶじょく)する' = to expose one's naked buttocks (in a public place) as an insult
I hope I explained okay.