WordNet sense 1 (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):

blackjack oak, jack oak, Quercus marilandica

WordNet sense 2 (a piece of metal covered by leather with a flexible handle; used for hitting people):

sap, cosh

WordNet sense 3 (a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship):

black flag, Jolly Roger, pirate flag

WordNet sense 4 (exert pressure on someone through threats):

blackmail, pressure

WordNet sense 5 (a gambling game using cards; the object is to hold cards having a higher count than those dealt to the bank up to but not exceeding 21):

twenty-one, vingt-et-un