joker
jok.er \'jo--k*r\ n 1a: a person given to joking 1b: FELLOW 2a: a playing
card added to a pack as a wild card or as the highest-ranking card 2b1: an
ambiguous or apparently immaterial clause inserted in a legislative bill
to make it inoperative or uncertain in some respect 2b2: an unsuspected,
misleading, or misunderstood clause, phrase, or word i n a document that
nullifies or greatly alters it 2c: something held in reserve to gain an end
or escape from a predicament 2d: an unsuspected or not readily apparent
fact, factor, or condition that thwarts or nullifies a seeming advantage