Proverbs 15:28
The heart of the righteous weighs its answers, but the mouth of the wicked gushes evil.

In another version it says, "The heart of the righteous studies how to answer". The righteous ones think beforehand what they should and will say. Their words are not based only on impulse and reaction.

(James 1:19-20) My dear brothers, take note of this: Everyone should be quick to listen, slow to speak and slow to become angry, for man's anger does not bring about the righteous life that God desires. (Prov 10:19 ) When words are many, sin is not absent, but he who holds his tongue is wise.

Application

We will do our best to speak words carefully. (Eph 4:29 NIV) 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.