Definition of jack:

part of speech: noun

A homely substitute for a coat of mail; a short loose coat terminating at the waist.

part of speech: noun

A name applied as an expression of familiarity; any mechanical contrivance for replacing the personal service of an attendant; a screw for raising heavy weights; a contrivance to turn a spit; any timber cut short of its usual length; a flag or ensign; a sailor - usually in composition, as jack- tar; a prefix signifying male, as jack- ass.