Webster's English Dictionary

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blackjack

1. black.jack \'blak-.jak\ n [black + jack (vessel)] 1: a tankard for beer 
   or ale usu. 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: a common often scrubby oak (Quercus 
   marilandica) of the southeaste rn and southern U.S. with black bark 5: 
   TWENTY-ONE 
2. blackjack vt : to strike with or as if with a blackjack