What does the Bible say about "bad" words? I know the Bible says not to let "unwholesome talk" come out of our mouths (Ephesians 4:29), but to my knowledge it doesn't speak specifically about "bad words."

In fact, to my knowledge, the English language (and especially American English) is somewhat unique in that it has words that are always considered bad, in any context. In my (limited) study of other languages (which is mostly Spanish), most "bad words" are perfectly good words, used in a specifically "bad" context.

Is it possible to use a "bad" word in a context that is not "unwholesome?"

I have a very close (Christian) friend, and he and I occasionally use "bad" words in our conversation, but always in a joking sense. Never out of anger or malice toward the other person. Is this okay, or should we change our behavior?
Ephesians 4:29 says:


  Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen.


Ephesians 5:4 says:


  Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving.


Language is a collective memory or cultural history. In other words, the meaning of words are defined by society. Per Ephesians 5:4, there could be no obscenity, foolishness or joking apart from an audience to interpret the message. Just as words in English have no meaning to people that don't speak English, the words can have certain meanings in certain sub-cultures or circles.

If certain words are not interpreted as obscene in a given circle, then the use of these words do not violate the command in Ephesians 5:4 and they will not have the appearance of evil per 1 Thessalonians 5:22. Same thing with Ephesians 4:29. The words are not sinful in themselves.

That said, in my experience it will be very tough for someone that incorporates this kind of language into their regular routine to refrain from using it around people that might not interpret it the same way. One way or another it always slips out. Furthermore, one has to be discerning enough not to cause another Christian to stumble by using this language improperly. It's probably safest not to use it and not flirt with violating these commands.