To tell someone to go on UrbanDictionary.com, and search up the word they don't know about (1). Can be shortened to "Urban it", if people know what your talking about, that is.
When a ignorant person asks what something means (usually in an inappropriate sense) and you don't even want to bother explaining it, you tell them to Urban Dictionary It, which means to look it up on urbandictionary.com. This is similar to google it.