A familiar nickname of, or substitute for, John. You are John Rugby, and you are Jack Rugby. Shak. 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. A popular colloquial name for a sailor; -- called also Jack tar, and Jack afloat. 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 device to pull off boots. A sawhorse or sawbuck. A machine or contrivance for turning a spit; a smoke jack, or kitchen jack. A compact, portable machine for planing metal. A machine for slicking or pebbling leather. A system of gearing driven by a horse power, for multiplying speed. A hood or other device placed over a chimney or vent pipe, to prevent a back draught. In the harpsichord, an intermediate piece communicating the action of the key to the quill; -- called also hopper. In hunting, the pan or frame holding the fuel of the torch used to attract game at night; also, the light itself. C. Hallock. 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. 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. The male of certain animals, as of the ass. A young pike; a pickerel. The jurel. A large, California rock fish (Sebastodes paucispinus); -- called also boccaccio, and mérou. The wall-eyed pike. A drinking measure holding half a pint; also, one holding a quarter of a pint. [Prov. Eng.] Halliwell. 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. 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. The knave of a suit of playing cards. A large wooden screw turning in a nut attached to the crosspiece of a rude press. -- Jack-in-office, an insolent fellow in authority. Wolcott. -- Jack-in-the-bush (Bot.), a tropical shrub with red fruit (Cordia Cylindrostachya). -- Jack-in-the-green, a chimney sweep inclosed in a framework of boughs, carried in Mayday processions. -- Jack-in-the-pulpit (Bot.), the American plant Arisæma triphyllum, or Indian turnip, in which the upright spadix is inclosed. -- Jack-of-the-buttery (Bot.), the stonecrop (Sedum acre). -- Jack-of-the-clock, a figure, usually of a man, on old clocks, which struck the time on the bell. -- Jack-on-both-sides, one who is or tries to be neutral. -- Jack-out-of-office, one who has been in office and is turned out. Shak. -- Jack the Giant Killer, the hero of a well-known nursery story. -- Jack-with-a-lantern, Jack-o'-lantern. (a) An ignis fatuus; a will-o'-the-wisp. "[Newspaper speculations] supplying so many more jack-o'-lanterns to the future historian." Lowell. (b) A lantern made of a pumpkin so prepared as to show in illumination the features of a human face, etc. -- Yellow Jack (Naut.), the yellow fever; also, the quarantine flag. See Yellow flag, under Flag.