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](http://www.biblegateway.com/passage/?search=Ephesians%204:29&version=NIV)) , 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?