Jack \Jack\, n. [F. Jacques James, L. Jacobus, Gr. ?, Heb. Ya 'aq[=o]b Jacob; prop., seizing by the heel; hence, a supplanter. Cf. Jacobite, Jockey.]

1. A familiar nickname of, or substitute for, John.

You are John Rugby, and you are Jack Rugby. --Shak.

2. An impertinent or silly fellow; a simpleton; a boor; a clown; also, a servant; a rustic. ``Jack fool.'' --Chaucer.

Since every Jack became a gentleman, There 's many a gentle person made a Jack. --Shak.

3. A popular colloquial name for a sailor; -- called also Jack tar, and Jack afloat.

4. A mechanical contrivance, an auxiliary machine, or a subordinate part of a machine, rendering convenient service, and often supplying the place of a boy or attendant who was commonly called Jack; as:
   (a) A device to pull off boots.
   (b) A sawhorse or sawbuck.
   (c) A machine or contrivance for turning a spit; a smoke jack, or kitchen jack.
   (b) (Mining) A wooden wedge for separating rocks rent by blasting.
   (e) (Knitting Machine) A lever for depressing the sinkers which push the loops down on the needles.
   (f) (Warping Machine) A grating to separate and guide the threads; a heck box.
   (g) (Spinning) A machine for twisting the sliver as it leaves the carding machine.
   (h) A compact, portable machine for planing metal.
   (i) A machine for slicking or pebbling leather.
   (k) A system of gearing driven by a horse power, for multiplying speed.
   (l) A hood or other device placed over a chimney or vent pipe, to prevent a back draught.
   (m) In the harpsichord, an intermediate piece communicating the action of the key to the quill; -- called also hopper.
   (n) In hunting, the pan or frame holding the fuel of the torch used to attract game at night; also, the light itself. --C. Hallock.

5. A portable machine variously constructed, for exerting great pressure, or lifting or moving a heavy body through a small distance. It consists of a lever, screw, rack and pinion, hydraulic press, or any simple combination of mechanical powers, working in a compact pedestal or support and operated by a lever, crank, capstan bar, etc. The name is often given to a jackscrew, which is a kind of jack.

6. The small bowl used as a mark in the game of bowls. --Shak.

Like an uninstructed bowler who thinks to attain the jack by delivering his bowl straight forward upon it. --Sir W. Scott.

7. The male of certain animals, as of the ass.

8. (Zo["o]l.)
   (a) A young pike; a pickerel.
   (b) The jurel.
   (c) A large, California rock fish (Sebastodes paucispinus); -- called also boccaccio, and m['e]rou.
   (d) The wall-eyed pike.

9. A drinking measure holding half a pint; also, one holding a quarter of a pint. [Prov. Eng.]

--Halliwell.

10. (Naut.)
   (a) A flag, containing only the union, without the fly, usually hoisted on a jack staff at the bowsprit cap; -- called also union jack. The American jack is a small blue flag, with a star for each State.
   (b) A bar of iron athwart ships at a topgallant masthead, to support a royal mast, and give spread to the royal shrouds; -- called also jack crosstree. --R. H. Dana, Jr.

Source: Webster's Revised Unabridged Dictionary (1913)