Fun:Four Letter Words
Four Letter Words in English are those that, due to their referents and easy pronouncability, are shunned by prudes, prigs and politicians (in public).
A short list will make it obvious to the reader why these words are usually considered taboo:
It should be noted that "four" and "word" are four letter words, as is "goat".
As a point of curiosity, there are 456.976 possible combinations of four letters, so even excluding the impossible ones gives a lot of room for play.