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also a wooden assistant for removing one’s boots (aka boot jack)

also a playing card, or a small metallic object with tetrahedrally oriented spikes used in the game Jacks https://en.wiktionary.org/wiki/jack#English

Jacks and jennies, male and female asses, jack leg, a bad tradesman, jacks a pointless kids game, jimmy jack, fiddle fart, ball the jack, go flat out