joker

Short definition from Wiktionary

Noun

1. 1
   A person who makes jokes.
2. 2
   A funny person.
3. 3
   A jester.
4. 4
   A playing card that features a picture of a joker (that is, a jester) and that may be used as a wild card in some card games.
5. 5
   Something kept in reserve that can be used to gain an advantage; a trump card.
6. 6
   An unspecified, vaguely disreputable person.
7. 7
   An unspecified, vaguely disreputable person: A loser.
8. 8
   A man.
9. 9
   A clause in a contract that undermines its apparent provisions.
10. 10
    A friendly unit that acts as a suspected hostile unit in a military excercise.
11. 11
    The option, in a pub quiz, of selecting one particular round in which one's team will score double points.
12. 12
    A fairy chess piece that moves like the last piece that was moved by the opponent.

Proper noun

1. 1
   A fictional villain of DC Comics, noted for his insanity and cunning, who either wears clown makeup or is disfigured to appear as such, and who is archenemy of Batman.