On th positive side we are to say
-good (Gk. agathos).
-words for edification, words that build up
-words that a needed
On the negative side he tells us to never let go from our mouths words that are in Greek sapos. It comes from the verb sepo from which where we get words like septic, or sepsis. It means to putrefy, to rot.
It is interesting that he doesn't use the verb speak, he uses the verb to let go or release. The image is that we have both kinds of words in our mouths. But we are only to let out the good ones, the ones that build up, the ones that people truly need. The rotten ones, the ones that tear down, the ones of no benefit to others. the ones no one needs to hear we should keep to ourselves. Just swallow them. Or better yet, don't let them form in your mouth. Stop them when they are just little thoughts.
But St. Paul knows human nature, those septic words and thoughts are going to form at some point in all of us, first in the mind, then in the mouth. If we are Christians, we filter. We let out of our mouths only the good words, the ones needed to build up. It takes incredible discipline. But there is no such thing as an undisciplined disciple.