Dirty, filthy (6)
I believe the answer is:
smutty
I believe this is a double definition.
'dirty' is the first definition.
(similar in meaning)
'filthy' is the second definition.
(I know that filthy can be written as smutty)
(Other definitions for smutty that I've seen before include "Rather rude" , "Blue" , "Mildly obscene" , "Rather indecent" , "Improper" .)