"Fish". I use the word as a general, all-purpose, substitute for swearing ('cos although I'm pretty open-minded, I'm still a bit of a prude when it comes to swearing). It serves for surprise, shock, joy, cheer, sadness, anger, and expressing contempt. "You fishing fishfisher, what do you fishing think you're fishing doing? Fish off!" Well, no, actually, I mostly just use it on its own ("Fish!"), which sometimes surprises people, although Tim's fairly used to it these days.
Reminds me of the A Night In</b> episode of Press Gang, where Tiddler says, "my mother says I'm not to use bad words, so I say 'fridge' whenever I want to say...'fridge'."