Quick answer
Righteous lips have a trained sense for the fitting word; the wicked mouth is fluent only in distortion. The chapter closes its speech theme on the note of instinct.
Read both lines
The arresting word is "know." Lips do not know things, except that after enough years they do: the righteous man's lips find the apt word, the welcome word, by feel, the way a musician's hands find the chord, the fruit of a heart long oriented toward what is good. The wicked mouth has been trained too, and its fluency is in the perverse; ask it for a report and the distortion arrives pre-installed. Neither mouth is improvising. That is the quiet warning and quiet hope of the whole chapter: every word you speak is also practice, and your mouth is becoming skilled at whatever you keep giving it to say.
Related: Proverbs 25:11 · Ephesians 4:29
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.