Belgium is the most revolting curseword imaginible. It was first used in the BBC Radio series, The Hitchhiker's Guide To the Galaxy, Secondary Phase
Because of its nature, it has no direct analog in American Slang.
In today’s modern Galaxy there is, of course, very little still held to be unspeakable.
Many words and expressions which only a matter of decades ago were considered so distastefully explicit that were they merely to be breathed in public, the perpetrator would be shunned, barred from polite society, and, in extreme cases, shot through the lungs; are now thought to be very healthy and proper, and their use in everyday speech is seen as evidence of a well-adjusted, relaxed, and totally un-frakked-up personality.
So, for instance, when in a recent national speech, the financial minister of the Royal World Estate of Qualvista actually dared to say that due to one thing and another, and the fact that no one had made any food for awhile and the king seemed to have died, and that most of the population had been on holiday now for over three years, the economy had now arrived at what he called, “One whole juju-flop situation,” everyone was so pleased he felt able to come out and say it, that they quite failed to notice that their five-thousand-year-old civilization had just collapsed overnight.
But though even words like “juju-flop,” “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 the stupid turlingdromes 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.