Wicked has two quite contradictory meanings. If something is pure evil, then it is wicked. Think Darth Vader. On the other hand, as an informal slang term, wicked also means excellent — as in "that DJ is wicked, man!" Go figure.
Other shades of meaning for wicked include something that is playful or enjoyably malicious. Someone might have a wicked sense of humor, for example, meaning it's fierce and takes aim sharply at its victims. Or it can also mean something that, while not actively evil, is still extremely unpleasant. Exams can be wicked, and so can someone's temper or the weather. The word comes from the Old English term wicca meaning "wizard."