dirty joke

A joke, story, or anecdote characterized by lewd, smutty, or otherwise indecent subject matter. My grandmother was the sweetest old lady imaginable, but she had this curious fondness for telling dirty jokes.

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.