The Collaborative International Dictionary of English v.0.48:

Rust \Rust\, v. t.
   1. To cause to contract rust; to corrode with rust; to affect
      with rust of any kind.
      [1913 Webster]

            Keep up your bright swords, for the dew will rust
            them.                                 --Shak.
      [1913 Webster]

   2. Fig.: To impair by time and inactivity. --Johnson.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rust \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.
      [1913 Webster]

   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.
      [1913 Webster]

   Note: Rust is also applied to many other minute fungi which
         infest vegetation, such as the species of Ustilago,
         Uredo, and Lecythea.
         [1913 Webster]

   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.
          [1913 Webster]

                Sacred truths cleared from all rust and dross of
                human mixtures.                   --Eikon
                                                  Basilike.
          [1913 Webster]

   Note: Rust is used in the formation of compounds of obvious
         meaning; as, rust-colored, rust-consumed, rust-eaten,
         and the like.
         [1913 Webster]

   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 (Zool.), a minute mite (Phytopius oleivorus)
      which, by puncturing the rind, causes the rust-colored
      patches on oranges.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rust \Rust\, v. i. [imp. & p. p. Rusted; p. pr. & vb. n.
   Rusting.] [AS. rustian.]
   1. To contract rust; to be or become oxidized.
      [1913 Webster]

            If gold ruste, what shall iron do?    --Chaucer.
      [1913 Webster]

            Our armors now may rust.              --Dryden.
      [1913 Webster]

   2. To be affected with the parasitic fungus called rust;
      also, to acquire a rusty appearance, as plants.
      [1913 Webster]

   3. Fig.: To degenerate in idleness; to become dull or
      impaired by inaction.
      [1913 Webster]

            Must I rust in Egypt? never more
            Appear in arms, and be the chief of Greece?
                                                  --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ferrugo \Fer*ru"go\ (f[e^]r*r[udd]"g[-o]), n. [L., iron rust,
   fr. ferrum iron.]
   A disease of plants caused by fungi, commonly called the
   rust, from its resemblance to iron rust in color.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rust
    adj 1: of the brown color of rust [syn: rust, rusty, rust-
           brown]
    n 1: a red or brown oxide coating on iron or steel caused by the
         action of oxygen and moisture
    2: a plant disease that produces a reddish-brown discoloration
       of leaves and stems; caused by various rust fungi
    3: the formation of reddish-brown ferric oxides on iron by low-
       temperature oxidation in the presence of water [syn: rust,
       rusting]
    4: any of various fungi causing rust disease in plants [syn:
       rust, rust fungus]
    v 1: become destroyed by water, air, or a corrosive such as an
         acid; "The metal corroded"; "The pipes rusted" [syn:
         corrode, rust]
    2: cause to deteriorate due to the action of water, air, or an
       acid; "The acid corroded the metal"; "The steady dripping of
       water rusted the metal stopper in the sink" [syn: corrode,
       eat, rust]
    3: become coated with oxide