William Burkitt on Ephesians 4:29

Here the apostle directs us how to manage our tongues, both negatively and positively, telling us what we should not speak, and what we should: Let no corrupt, rotten, filthy discourse, come out of your mouth; such as have rotten lungs have a stinking breath; filthy discourse argues a polluted heart; such noisome discourse is unsavoury to an holy ear, and greatly offensive, contagious, and infecting to common and ordinary hearers.

Next, he tells them what they should speak: That which is profitable and edifying, and that which may minister increase of grace to the hearers. Our speech should be so gracious and savoury, seasoned with salt, Col 4:6. Truth, holiness, and prudence, is the salt of our words; Christians must not suffer their tongues to run at random in their ordinary discourse; it is not sufficient that they do not speak to evil purposes, but they must speak to edifying purpose; that which has a tendency to make the hearers some way or other either wiser or better, this the apostle calls that which is good to the use of edifying.