The phrase four-letter word refers to a set of English words written with four letters which are considered profane, including common popular or slang terms for excretory functions, sexual activity, and genitalia. The "four-letter" claim refers to the fact that most English swear words are monosyllabic, and therefore are likely to have four letters on average. This euphemism came into use during the first half of the twentieth century.

From wikipedia.