Stay out of trouble. This slangy phrase began to be heard in Britain in the late nineteenth century and crossed the Atlantic soon afterward. Why cleanliness should be invoked is not known. Presumably keeping one’s nose clean would be equivalent to not dirtying it by poking it into someone else’s business (see poke one’s nose into). Anyhow, the term was very common by the 1940s, when it appeared in such popular novels as Manning Long’s False Alarm (1943): “Keep your nose clean and you’ll keep out of trouble.”