The whole business of swearing, especially English swearing, is mysterious. Of its very nature swearing is as irrational as magic-- indeed, it is a species of magic. But there is also a paradox about it, namely this: Our intention in swearing is to shock and wound, which we do by mentioning something that should be kept secret--usually something to do with the sexual functions. But the strange thing is that when a word is well established as a swear word, it seems to lose its original meaning; that is, it loses the thing that made it into a swear word.
