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.
2. (Mining) Bad, soft coal, containing much earthy matter,
found in the immediate locality of faults.
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.
4. Obscene language; ribaldry; obscenity.
He does not stand upon decency . . . but will talk
smut, though a priest and his mother be in the room.
Smut mill, a machine for cleansing grain from smut.