Lyric discussion by MattofDerbyshire 

To me I think that he's singing about a friend of his who is in an on-off relationship with an arsehole, maybe he likes whoever she is, maybe she doesn't want him as he's a creep (see what I did there) etc.

Thom doesn't like the bloke, referring to him as a stink. There are sexual innuendoes aplenty in this song: "Comes like a comet" "Suckered you but not your friends" The holy cow bit may be saying that this bloke will try to convince the girl that she is the love of his life.

"Changed the locks three times" another innuendo. He'll teach you how to get to purest hell is drawn as a parallel to holy cow, so whatever he promises her, is actually a lie, perhaps.

The chorus shows that she brings it upon herself, and that she should cut this bloke from her life, but doesn't and "that's what really hurts." Which is why I think he has feelings for her, whoever she may be.

Quite likely I'm completely wrong, soz Thom (and the rest of Radiohead), but that's how this song comes across to me.

An error occured.