\Rust\, n. [AS. rust; akin to D. roest, G. & Sw. rost,
Icel. ry[eth]; -- named from its color, and akin to E. red.
[root]113. See {Red}.]
1. (Chem.) The reddish yellow coating formed on iron when
   exposed to moist air, consisting of ferric oxide or
   hydroxide; hence, by extension, any metallic film of
   corrosion.
2. (Bot.) A minute mold or fungus forming reddish or rusty
   spots on the leaves and stems of cereal and other grasses
   ({Trichobasis Rubigo-vera}), now usually believed to be a
   form or condition of the corn mildew ({Puccinia
   graminis}). As rust, it has solitary reddish spores; as
   corn mildew, the spores are double and blackish.
Note: Rust is also applied to many other minute fungi which
      infest vegetation, such as the species of {Ustilago},
      {Uredo}, and {Lecythea}.
3. That which resembles rust in appearance or effects.
   Specifically: (a) A composition used in making a rust
   joint. See {Rust joint}, below.
   (b) Foul matter arising from degeneration; as, rust on
       salted meat.
   (c) Corrosive or injurious accretion or influence.
             Sacred truths cleared from all rust and dross of
             human mixtures.                   --Eikon
                                               Basilike.
Note: Rust is used in the formation of compounds of obvious
      meaning; as, rust-colored, rust-consumed, rust-eaten,
      and the like.
{Rust joint}, a joint made between surfaces of iron by
   filling the space between them with a wet mixture of
   cast-iron borings, sal ammoniac, and sulphur, which by
   oxidation becomes hard, and impervious to steam, water,
   etc.
{Rust mite} (Zo["o]l.), a minute mite ({Phytopius oleivorus})
   which, by puncturing the rind, causes the rust-colored
   patches on oranges.

\Rust\, v. i. [imp. & p. p. {Rusted}; p. pr. & vb. n.
{Rusting}.] [AS. rustian.]
1. To contract rust; to be or become oxidized.
         If gold ruste, what shall iron do?    --Chaucer.
         Our armors now may rust.              --Dryden.
2. To be affected with the parasitic fungus called rust;
   also, to acquire a rusty appearance. as plants.
3. Fig.: To degenerate in idleness; to become dull or
   impaired by inaction.
         Must I rust in Egypt? never more Appear in arms, and
         be the chief of Greece?               --Dryden.

\Rust\, v. t.
1. To cause to contract rust; to corrode with rust; to affect
   with rust of any kind.
         Keep up your bright swords, for the dew will rust
         them.                                 --Shak.
2. Fig.: To impair by time and inactivity. --Johnson.