Something ribald is funny, but in a vulgar or off-color way. If someone makes a ribald joke in class, she will probably be sent to the principal's office.
The word ribald can be used to describe a joke, a movie, or even a person, like a ribald comedian on a late-night talk show. Ribald humor is generally considered to be inappropriate around children, and if you make a ribald comment, you might inadvertently offend someone. This word derives from an Old French word meaning "rogue" or "debauched."