late 14c., jakke "a mechanical device," from the masc. name Jack (q.v.). The proper name was used in Middle English for "any common fellow" (see jack (n.1)) and thereafter extended, alone or in compound, to various mechanical contrivances or appliances which do the work of common servants and assistants or are liable to rough use (1570s, in reference to a machine for turning the spit in roasting meat); for this use compare billy, jimmy, Jenny. Jack also was used generically in reference to male animals (1620s, see jackass, jackdaw, etc.).
Specifically as a contrivance for raising great weights by force exerted from below, it is attested by 1703. As the name of a device for pulling off boots, from 1670s. The slang meaning "money" is recorded by 1890 (in earlier slang it meant "a small coin"). In telephony and telegraphy, "a terminal by means of which instruments can be plugged into a circuit," it is attested by 1891, in reference to the spring-jack switch in it.
For the playing card, see jack (n.3). The jack in Union Jack is a nautical term for "small flag at the bow of a ship" (1630s) and perhaps is from the word's secondary sense of "smaller than normal size."
