smut 
(smŭt)
(smŭt)n.
1.
a. A particle of dirt or soot: "the smoke and flames blowing from the lantern which peppered him with large black smuts" (Joyce Cary).
b. A smudge made by soot, smoke, or dirt.
2. Sexually explicit or prurient language or material, such as books or movies.
3. See smut fungus.
v. smut·ted, smut·ting, smuts
v.tr.
1. To blacken or smudge, as with smoke or grime.
2. To affect (a plant) with smut fungus.
v.intr.
To be or become blackened or smudged.
[From Middle English smotten, smutten, to defile.]
smutti·ly adv.
smutti·ness n.
smutty adj.