Source : Webster's Revised Unabridged Dictionary (1913)

   {Jack rabbit} (Zo["o]l.), any one of several species of large
      American hares, having very large ears and long legs. The
      California species ({Lepus Californicus}), and that of
      Texas and New Mexico ({L. callotis}), have the tail black
      above, and the ears black at the tip. They do not become
      white in winter. The more northern prairie hare ({L.
      campestris}) has the upper side of the tail white, and in
      winter its fur becomes nearly white.

   {Jack rafter} (Arch.), in England, one of the shorter rafters
      used in constructing a hip or valley roof; in the United
      States, any secondary roof timber, as the common rafters
      resting on purlins in a trussed roof; also, one of the
      pieces simulating extended rafters, used under the eaves
      in some styles of building.

   {Jack salmon} (Zo["o]l.), the wall-eyed pike, or glasseye.

   {Jack sauce}, an impudent fellow. [Colloq. & Obs.]

   {Jack shaft} (Mach.), the first intermediate shaft, in a
      factory or mill, which receives power, through belts or
      gearing, from a prime mover, and transmits it, by the same
      means, to other intermediate shafts or to a line shaft.

   {Jack sinker} (Knitting Mach.), a thin iron plate operated by
      the jack to depress the loop of thread between two
      needles.

   {Jack snipe}. (Zo["o]l.) See in the Vocabulary.

   {Jack staff} (Naut.), a staff fixed on the bowsprit cap, upon
      which the jack is hoisted.

   {Jack timber} (Arch.), any timber, as a rafter, rib, or
      studding, which, being intercepted, is shorter than the
      others.

   {Jack towel}, a towel hung on a roller for common use.

   {Jack truss} (Arch.), in a hip roof, a minor truss used where
      the roof has not its full section.

   {Jack tree}. (Bot.) See 1st {Jack}, n.

   {Jack yard} (Naut.), a short spar to extend a topsail beyond
      the gaff.

   {Blue jack}, blue vitriol; sulphate of copper.

   {Hydraulic jack}, a jack used for lifting, pulling, or
      forcing, consisting of a compact portable hydrostatic
      press, with its pump and a reservoir containing a supply
      of liquid, as oil.

   {Jack-at-a-pinch}.
       (a) One called upon to take the place of another in an
           emergency.
       (b) An itinerant parson who conducts an occasional
           service for a fee.

   {Jack-at-all-trades}, one who can turn his hand to any kind
      of work.

   {Jack-by-the-hedge} (Bot.), a plant of the genus {Erysimum}
      ({E. alliaria}, or {Alliaria officinalis}), which grows
      under hedges. It bears a white flower and has a taste not
      unlike garlic. Called also, in England, {sauce-alone}.
      --Eng. Cyc.

   {Jack-in-a-box}.
       (a) (Bot.) A tropical tree ({Hernandia sonora}), which
           bears a drupe that rattles when dry in the inflated
           calyx.
       (b) A child's toy, consisting of a box, out of which,
           when the lid is raised, a figure springs.
       (c) (Mech.) An epicyclic train of bevel gears for
           transmitting rotary motion to two parts in such a
           manner that their relative rotation may be variable;
           applied to driving the wheels of tricycles, road
           locomotives, and to cotton machinery, etc.; an
           equation box; a jack frame; -- called also
           {compensating gearing}.
       (d) 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 {Aris[ae]ma
      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}.

Source : WordNet®

yellow jack
     n 1: caused by a flavivirus transmitted by a mosquito [syn: {yellow
          fever}, {black vomit}]
     2: yellow flag hoist on a ship in quarantine
     3: fish of western Atlantic and Gulf of Mexico [syn: {Caranx
        bartholomaei}]