dirty word

1. An obscene or vulgar word.
2. A word, expression, or concept viewed as odious or intolerable: a feisty campaigner to whom “give up” was a dirty word.

A smutty story, as in Teenagers love to tell dirty jokes. The use of dirty in the sense of "obscene" or "indecent" dates from the late 1500s, but its application to a joke began only in the 20th century. The same sense also appears in such expressions as dirty book, for a pornographic book; dirty old man, for a middle-aged or elderly lecher; and dirty word or talk dirty, for a lewd, offensive expression or sexually explicit conversation.

The noun has one meaning:

Meaning #1: an indelicate joke
  Synonyms: dirty story, blue joke, blue story