When he is lying on his stomach and you kiss softly down his back, then as you work your way down, you fist his asshole and the other girl who happens to be there watching slaps him in the face with a fish as he says “aurghh!”
Girl1: Last night when my man asked for a threesome, we gave him the seal instead!!
Girl2: oh no you didn't!!
A variation of the phrase 'breaking the seal' whereby one often has to go to the toilet frequently when drinking, once they have gone for the first time of the evening. However, this version can have a variety of misunderstandings that sound like one is about to commit/has just committed an animal rights abuse.
Used mostly in parts of Lancashire and Cumbria, and in particular Lancaster.