Coprolalia

coprolalia \ˌkä-prə-ˈlā-lē-ə\ noun: obsessive or uncontrollable use of obscene language

Etymology: New Latin, from Greek kopr-, kopro-, from kopros dung, feces + lalia chatter, from lalein to chat

This term gives credence to that old threat: "I'm going to wash your mouth out with soap if you keep up that language!"