The Collaborative International Dictionary of English v.0.48:

Smut \Smut\, n. [Akin to Sw. smuts, Dan. smuds, MHG. smuz, G.
   schmutz, D. smet a spot or stain, smoddig, smodsig,
   smodderig, dirty, smodderen to smut; and probably to E.
   smite. See Smite, v. t., and cf. Smitt, Smutch.]
   1. Foul matter, like soot or coal dust; also, a spot or soil
      made by such matter.
      [1913 Webster]

   2. (Mining) Bad, soft coal, containing much earthy matter,
      found in the immediate locality of faults.
      [1913 Webster]

   3. (Bot.) An infection of cereal grains producing a swelling
      which is at length resolved into a powdery sooty mass. It
      is caused by parasitic fungi of the genus Ustilago.
      Ustilago segetum, or Ustilago Carbo, is the commonest
      kind; that of Indian corn is Ustilago maydis.
      [1913 Webster]

   4. Obscene language; ribaldry; obscenity.
      [1913 Webster]

            He does not stand upon decency . . . but will talk
            smut, though a priest and his mother be in the room.
                                                  --Addison.
      [1913 Webster]

   Smut mill, a machine for cleansing grain from smut.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ustilago maydis
    n 1: a common smut attacking Indian corn causing greyish white
         swellings that rupture to expose a black spore mass [syn:
         boil smut, Ustilago maydis]