What does 420 mean in dirty?

Excuse me, could you possibly clarify the meaning of "420" when used in a colloquial or slang context that might be considered risqué or inappropriate? I've heard it mentioned in certain circles but have been unable to pin down a definitive explanation. Could it be a code word, a reference to a particular activity, or perhaps a nod to a specific culture or subculture? Any insight you could provide would be greatly appreciated.