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