In the US, it usually means a mischievous child, and not really in a negative way, like Michael said. There was a TV series back in the 30's (I think): 'The Little Rascals'. And Elmer Fudd would always call Bugs Bunny 'that wascawwy wabbit'. (from the Warner Brother's cartoons, he couldn't pronounce the 'w's..) So, occasionally, at least, it can also mean cunning and able to outwit someone.

Also, it's especially used to refer to small children who get into trouble, misbehave, and so on. Often it's used jokingly as a general term for a small kid with a lot of energy without implying anything negative. When said about adults, it's more of an insult but not very commonly used.

It's a bad/abusive word. A wicked or evil person; someone who does evil deliberately.