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"club," 1848, American English, originally burglars' slang for "crowbar;" meaning "policeman's club" first recorded 1856, probably from nickname of William, applied to various objects (cf. jack, jimmy, jenny).
A club or truncheon, now esp one carried by the police •Associated with the police fr the 1850s, but a reference of the 1880s still describes only what would now be called a blackjack, definitely a criminal's weapon
[1840s+; said to be a burglar's pet or secret name for his crowbar, along with Jemmy or Jimmy; he also used it as a weapon]