n.1 a person who jokes.
2 sl. a fellow; a man.
3 a playing-card usu. with a figure of a jester, used in some games esp. as a wild card.
4 US a clause unobtrusively inserted in a bill or document and affecting its operation in a way not immediately apparent.
5 an unexpected factor or resource.
Phrases and idioms:
the joker in the pack an unpredictable factor or participant.