Definition of "Joker" :

noun: a playing card that is usually printed with a picture of a jester

noun: 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."

noun: a person who enjoys telling or playing jokes

noun: a person who does something thoughtless or annoying

"Some joker is blocking the driveway."