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."