JOKER

noun Date: 1729 1. a person given to joking ; wag , fellow , guy , a playing card added to a pack as a wild card or as the highest-ranking card, 3. a. an ambiguous or apparently immaterial clause inserted in a legislative bill to make it inoperative or uncertain in some respect, an unsuspected, misleading, or misunderstood clause, phrase, or word in a document that nullifies or greatly alters it, something (as an expedient or stratagem) held in reserve to gain an end or escape from a predicament, an unsuspected or not readily apparent fact, factor, or condition that thwarts or nullifies a seeming advantage