There is no such thing as "a bad work",
because some genius took the meaning of a normal word,
and made it into something it is not,
the majority of this world will never see it that way though.
When referring to one's own buttocks, the word "ass" is fine. However, when used in an insulting manner towards others (eg John is an ass), it is offensive.
Also, this is an amazingly stupid question (no offense)
It all depends on how you are using the, like "what a pain in the ass" using it that way is not bad. But when using it as an insult it can be called a bad word.