In the US version of the book, it's Belgium. In the UK version it's f*ck. I can't recall what's in the radio show, if anything.

The radio show has belgium: Zaphod says it when falling out of the statue of the cup, on the shoe planet. The book doesn't do most of the shoe planet material, but the belgium speech got put in the US version for the bit at the flying party where the actress has just won the Rory award for the most gratuitous use of the word {f*ck,belgium} in a serious screenplay.

Peter