Source : Webster's Revised Unabridged Dictionary (1913)

Iron \I"ron\ ([imac]"[u^]rn), a. [AS. [=i]ren, [=i]sen. See
   {Iron}, n.]
   1. Of, or made of iron; consisting of iron; as, an iron bar,
      dust.

   2. Resembling iron in color; as, iron blackness.

   3. Like iron in hardness, strength, impenetrability, power of
      endurance, insensibility, etc.; as:
      (a) Rude; hard; harsh; severe.

                Iron years of wars and dangers.   --Rowe.

                Jove crushed the nations with an iron rod.
                                                  --Pope.
      (b) Firm; robust; enduring; as, an iron constitution.
      (c) Inflexible; unrelenting; as, an iron will.
      (d) Not to be broken; holding or binding fast; tenacious.
          ``Him death's iron sleep oppressed.'' --Philips.

   Note: Iron is often used in composition, denoting made of
         iron, relating to iron, of or with iron; producing
         iron, etc.; resembling iron, literally or figuratively,
         in some of its properties or characteristics; as,
         iron-shod, iron-sheathed, iron-fisted, iron-framed,
         iron-handed, iron-hearted, iron foundry or
         iron-foundry.

   {Iron age}.
      (a) (Myth.) The age following the golden, silver, and
          bronze ages, and characterized by a general
          degeneration of talent and virtue, and of literary
          excellence. In Roman literature the Iron Age is
          commonly regarded as beginning after the taking of
          Rome by the Goths, A. D. 410.
      (b) (Arch[ae]ol.) That stage in the development of any
          people characterized by the use of iron implements in
          the place of the more cumbrous stone and bronze.

   {Iron cement}, a cement for joints, composed of cast-iron
      borings or filings, sal ammoniac, etc.

   {Iron clay} (Min.), a yellowish clay containing a large
      proportion of an ore of iron.

   {Iron cross}, a Prussian order of military merit; also, the
      decoration of the order.

   {Iron crown}, a golden crown set with jewels, belonging
      originally to the Lombard kings, and indicating the
      dominion of Italy. It was so called from containing a
      circle said to have been forged from one of the nails in
      the cross of Christ.

   {Iron flint} (Min.), an opaque, flintlike, ferruginous
      variety of quartz.

   {Iron founder}, a maker of iron castings.

   {Iron foundry}, the place where iron castings are made.

   {Iron furnace}, a furnace for reducing iron from the ore, or
      for melting iron for castings, etc.; a forge; a
      reverberatory; a bloomery.

   {Iron glance} (Min.), hematite.

   {Iron hat}, a headpiece of iron or steel, shaped like a hat
      with a broad brim, and used as armor during the Middle
      Ages.

   {Iron horse}, a locomotive engine. [Colloq.]

   {Iron liquor}, a solution of an iron salt, used as a mordant
      by dyers.

   {Iron man} (Cotton Manuf.), a name for the self-acting
      spinning mule.

   {Iron} {mold or mould}, a yellow spot on cloth stained by
      rusty iron.

   {Iron ore} (Min.), any native compound of iron from which the
      metal may be profitably extracted. The principal ores are
      magnetite, hematite, siderite, limonite, G["o]thite,
      turgite, and the bog and clay iron ores.

   {Iron pyrites} (Min.), common pyrites, or pyrite. See
      {Pyrites}.

   {Iron sand}, an iron ore in grains, usually the magnetic iron
      ore, formerly used to sand paper after writing.

   {Iron scale}, the thin film which on the surface of wrought
      iron in the process of forging. It consists essentially of
      the magnetic oxide of iron, {Fe3O4>}.

   {Iron works}, a furnace where iron is smelted, or a forge,
      rolling mill, or foundry, where it is made into heavy
      work, such as shafting, rails, cannon, merchant bar, etc.



   {Horse emmet} (Zo["o]l.), the horse ant.

   {Horse finch} (Zo["o]l.), the chaffinch. [Prov. Eng.]

   {Horse gentian} (Bot.), fever root.

   {Horse iron} (Naut.), a large calking iron.

   {Horse latitudes}, a space in the North Atlantic famous for
      calms and baffling winds, being between the westerly winds
      of higher latitudes and the trade winds. --Ham. Nav.
      Encyc.

   {Horse mackrel}. (Zo["o]l.)
      (a) The common tunny ({Orcynus thunnus}), found on the
          Atlantic coast of Europe and America, and in the
          Mediterranean.
      (b) The bluefish ({Pomatomus saltatrix}).
      (c) The scad.
      (d) The name is locally applied to various other fishes,
          as the California hake, the black candlefish, the
          jurel, the bluefish, etc.

   {Horse marine} (Naut.), an awkward, lubbery person; one of a
      mythical body of marine cavalry. [Slang]

   {Horse mussel} (Zo["o]l.), a large, marine mussel ({Modiola
      modiolus}), found on the northern shores of Europe and
      America.

   {Horse nettle} (Bot.), a coarse, prickly, American herb, the
      {Solanum Carolinense}.

   {Horse parsley}. (Bot.) See {Alexanders}.

   {Horse purslain} (Bot.), a coarse fleshy weed of tropical
      America ({Trianthema monogymnum}).

   {Horse race}, a race by horses; a match of horses in running
      or trotting.

   {Horse racing}, the practice of racing with horses.

   {Horse railroad}, a railroad on which the cars are drawn by
      horses; -- in England, and sometimes in the United States,
      called a {tramway}.

   {Horse run} (Civil Engin.), a device for drawing loaded
      wheelbarrows up an inclined plane by horse power.

   {Horse sense}, strong common sense. [Colloq. U.S.]

   {Horse soldier}, a cavalryman.

   {Horse sponge} (Zo["o]l.), a large, coarse, commercial sponge
      ({Spongia equina}).

   {Horse stinger} (Zo["o]l.), a large dragon fly. [Prov. Eng.]
      

   {Horse sugar} (Bot.), a shrub of the southern part of the
      United States ({Symplocos tinctoria}), whose leaves are
      sweet, and good for fodder.

   {Horse tick} (Zo["o]l.), a winged, dipterous insect
      ({Hippobosca equina}), which troubles horses by biting
      them, and sucking their blood; -- called also {horsefly},
      {horse louse}, and {forest fly}.

   {Horse vetch} (Bot.), a plant of the genus {Hippocrepis} ({H.
      comosa}), cultivated for the beauty of its flowers; --
      called also {horsehoe vetch}, from the peculiar shape of
      its pods.

   {Iron horse}, a locomotive. [Colloq.]

   {Salt horse}, the sailor's name for salt beef.

   {To look a gift horse in the mouth}, to examine the mouth of
      a horse which has been received as a gift, in order to
      ascertain his age; -- hence, to accept favors in a
      critical and thankless spirit. --Lowell.

   {To take horse}.
      (a) To set out on horseback. --Macaulay.
      (b) To be covered, as a mare.
      (c) See definition 7 (above).

Source : WordNet®

iron horse
     n : (c. 1840) an early term for a locomotive