I'm looking for a good translation of the word dirty, in the sense of
Morally unclean; obscene or indecent, especially sexually. (Wiktionary)
Ein __ Witz
Which word could I use?
Another related alternative would be “versaut”, literally “spoiled”, and with much the same connotation.
The difference between the German “versaut” and English “spoiled” – and the reason why it can also be used in the sense of “dirty” – is this: A “spoiled joke” is a joke badly told. But a “versauter Witz” (usually) means a dirty joke.
(Likewise, you can’t say “das ist eine versaute Frucht” to warn somebody off eating a spoiled apple.)