Basic Guidelines For English Spellings
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An attendant or retainer kept by a nobleman or landowner; an armed servant.
Mid 16th century; earliest use found in John Knox (c1514–1572), religious reformer. Either (i) from jack + man, or (ii) from Jack + man.
A person who operates a jack. Now: especially a person who operates a lifting jack.
Early 20th century; earliest use found in The Federal Reporter. From Jack + man.