The approximate 2 - 10 seconds of immense pain inside of a female's anus that comes and goes without warning. It feels like - more or less - a knife in one's ass.
Girl 1: *starts to wince in pain whilst squirming in chair*
Girl 2: Are you alright?!
Girl 1: *stops* Yeah, I just had a massive case of knife ass
Girl 2: Aw yeah I hate that
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.