Merriam-Webster Collegiate Dictionary - blackjack

 I. noun  Date: 1591  1. black + jack (vessel) a tankard for beer or ale usually of tar-coated leather  2. sphalerite  3. a hand weapon typically consisting of a piece of leather-enclosed metal with a strap or springy shaft for a handle  4. an often scrubby oak (Quercus marilandica) chiefly of the southeastern U.S. — called also ~ oak  5. a card game the object of which is to be dealt cards having a higher count than those of the dealer up to but not exceeding 21 — called also twenty-one, vingt-et-un  II. transitive verb  Date: 1905  1. to strike with a ~  2. to coerce with threats or pressure