According to "The Hitchhiker's Guide to the Galaxy", what is the dirtiest word in the galaxy?

But though even words like 'joojooflop', 'swut' and 'turlingdrome' are now perfectly acceptable in common usage, there is one word that is still beyond the pale. The concept it embodies is so revolting that the publication or broadcast of the word is utterly forbidden in all parts of the Galaxy except one...where they don't know what it means. That word is 'belgium' and it is only ever used by loose tongued people like Zaphod Beeblebrox in situations of dire provocation. Such as...

Ford Prefect: And I'll tell you another interesting thing...

Zaphod: Look, I don't want to be interested! I don't wanna be relaxed or stimulated or have my horizons broadened, I just wanna be rescued, Ford! [Panicked breathing - quite understandable, given that he's barely hanging on above a drop of some thirteen miles!] I just wanna be swutting well RESCUED!

Ford Prefect: I'm sorry, I've told you: No way.

Zaphod: Aw, belgium, man, BELGIUM!