A Scandinavian expletive. It is often used as a catch-all swear, although it is allegedly very vulgar. There is no literal translation to English. A Norwegian friend told me that it was roughly equivalent to saying "God damn!" in English.

Also written fy Fan. Not very vulgar. Rather blasphemous though - basically uttering one of the names Swedes have for Satan.
Excessive use has watered down this expletive in some cliques of society, making it usable at most occasions.