joker

Etymology

joke + -er, but in the sense of a playing card possibly by alteration of Jucker, also the origin of the name of the card game euchre.

noun

1. A person who makes jokes.
2. (slang) A funny person.
3. A jester.
4. 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.
5. An unspecified, vaguely disreputable person.
   Some joker keeps throwing eggs at my windows.
6. (New Zealand, colloquial) A man.
7. A clause in a contract that undermines its apparent provisions.
   Discussion of contracts and the many provisions contained therein led to a vote making it the sense of the convention that manufacturers should use a simple sales contract, free from jokers. 1922, Farm Machinery and Equipment, page lxxxiii
   Then, sir, on page 12 of the agreement there is a joker clause, which provides for payments in addition to the ten per cent, […] 1939, Canadian Parliament, Official Report of Debates, House of Commons, volume 218, page 858
   Stone claimed that there was a Joker in the contract, one clause (No. 2) calling for two weeks' notice and another (No. 8) calling for payment on a par-day basis after the first two weeks. 1942, Billboard, volume 54, number 41, page 5
   The object of these provisions is to prevent insertion of "jokers" or "sleepers" in bills and securing passage under the false color of the title. 1958, Duncan Leroy Kennedy, Bill drafting, page 12
8. (military) A friendly unit that acts as a suspected hostile unit in a military excercise.
   Joker - A friendly track or contact acting as a "suspect" track for exercise purposes only. (STANAG 1241) 1998, APP-6A: Military Symbols for Land-Based Systems, page 9