joker

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who tells or plays jokes.
• noun An insolent person who seeks to make a show of cleverness.
• noun Informal An annoying or inept person.
• noun Games A playing card, usually printed with a picture of a jester, used in certain games as the highest-ranking card or as a wild card.
• noun A clause that is included in a legislative bill or a contract in order to render the bill or contract inoperative or oppressive in some respect while appearing to be innocuous until the harm has been done.
• noun An unforeseen but important difficulty, fact, or circumstance.
• noun A deceptive means of getting the better of someone.

from The Century Dictionary.

• noun One who jokes, in speech or in deed; a jester; a merry fellow.
• noun A playing-card, either blank or having some comical or other special device, added to a pack, and used in some games, as in euchre. It is always a trump, and generally the highest trump. Often called jolly joker.
• noun Figuratively, something concealed that wins the game; a trick under an innocent guise: as, a joker in a legislative bill.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who makes jokes or jests.
• noun (Card Playing) See Best bower, under 2d Bower.
• noun (Card Playing) An extra card usually included in a deck of playing cards, having the same design as the others on the back, but on the face having a picture of a jester. It is not included in the deck used in most games, but in certain games may be included and then takes on a special value, such as the highest-valued card, or a wild card.
• noun A clause placed in a document, such as a contract or a piece of legislation, not itself appearing significant, but in a subtle way substantially changing the effect of the document.
• noun Any fact or condition which is unknown or not apparent, which reverses an apparently advantageous position; a kicker.
• noun A person; a fellow; a chap; -- usually used in a mildly disparaging sense.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person who makes jokes.
• noun slang A funny person.
• noun A jester.
• noun 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.
• noun An unspecified, vaguely disreputable person.
• noun New Zealand, colloquial A man.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an inconspicuous clause in a document or bill that affects its meaning in a way that is not immediately apparent
• noun a person who enjoys telling or playing jokes
• noun a playing card that is usually printed with a picture of a jester
• noun a person who does something thoughtless or annoying