Etymology
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blackjack (n.)

used in various senses since 16c., earliest is possibly "tar-coated leather jug for beer" (1590s), from black (adj.) + jack in any of its many slang meanings. From 1867 as "pirate flag." The hand-weapon so called from 1889; the card game by 1900.

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Definitions of blackjack
1
blackjack (n.)
a common scrubby deciduous tree of central and southeastern United States having dark bark and broad three-lobed (club-shaped) leaves; tends to form dense thickets;
Synonyms: blackjack oak / jack oak / Quercus marilandica
blackjack (n.)
a piece of metal covered by leather with a flexible handle; used for hitting people;
Synonyms: cosh / sap
blackjack (n.)
a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship;
Synonyms: black flag / pirate flag / Jolly Roger
blackjack (n.)
a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21;
Synonyms: twenty-one / vingt-et-un
2
blackjack (v.)
exert pressure on someone through threats;
Synonyms: blackmail / pressure
From wordnet.princeton.edu