The word smutty came from pure-hearted farmers. Really.
Smut is the name of a mold that grows in corn ears. The mold breaks off into fine dirt-like powder when the corn is shucked.
Therefore the word smut became synonymous with dirt, or dirty.
When dirty came to describe sexual talk or writing, calling the same behavior “smutty” seemed to make it even more reviled.
These days, smut usually refers to obscene content in the media or in language. If you’re into smut on the internet, you should stop now before you go blind.
Smutty can also refer to yet another PHP MVC based framework, as seen here http://code.google.com/p/smutty/