Enochian doesn't have any curse words, you can say some pretty awful insults that are just as bad but there isn't a single word that does it.

Castiel didn't really understand why humans got so upset at a few meaningless words but one day after an entire day of Michael frustrating him a curse just slips past his lips. Lucifer is amused by it but Michael hates hearing his little brother speak like that.After that Castiel uses curse words every time Michael is being particularly jerk, just to spite him.

Can be Castiel/Michael, Castiel/Lucifer (or both!) or gen (or both!!).