joker in WordNet English dictionary

1. a playing card that is usually printed with a picture of a jester
   joker 
   
2. an inconspicuous clause in a document or bill that affects its meaning in a way that is not immediately apparent
   "when I demanded my money he showed me the joker in the contract"
   joker 
   
3. a person who enjoys telling or playing jokes
   joker  jokester 
   
4. a person who does something thoughtless or annoying
   "some joker is blocking the driveway"
   joker  turkey