The Hitchhiker's Guide wiki states that 'Belgium' is the rudest word in the galaxy. Why?
Back in the late 70s/early 80s in the UK there was a meme doing the rounds that Belgium was the most boring country in Europe. For example there was a joke:
Name five famous Belgians - not including Hercule Poirot
Poirot is, of course, fictional - the point being there are no well know famous Belgians¹.
I would guess, though I know of no documentary evidence for this, that this is why Adams chose Belgium as the rudest word. It fits with his taste for the absurd.
¹ before I get flamed, this says more about the English than the Belgians
I read somewhere that Adams ran into problems with his US publisher in 1979 for including a reference to
A Rory award for the most gratuitous use of the word "fuck" in a serious screenplay
He was forced by the publisher to change this, and chose the innocuous word Belgium instead.
He then got his "revenge" by writing about Belgium being actually the most offensive word in the universe, allowing him to suggest he hadn't given in, but had actually made it worse.
I consider it likely that the meme started with the "Prejudice" sketch from Monty Python. It was broadcast first in 1973.
The choice of "Belgians" could well be arbitrary here as the sketch suggests being only one of a series.