The Hebrew calls him a witness of Belial, footnoted in the NKJV as a worthless witness.
Quick answer
A corrupt witness makes a mockery of the very thing a court exists for. The second line says he has an appetite for it.
Read both lines
The word behind "justice" is mishpat, the same term that carries the poverty-and-injustice thread from 13:23 onward, so the offense here is not merely lying, it is treating the machinery that protects people as a joke. The second line changes the metaphor from a courtroom to a mouth that swallows, and the image is of consumption rather than accident: this man does not slip into iniquity, he feeds on it. The chapter has been building toward this since verse 5, and the next verse names what is waiting. Perjury looks like a speech problem and is actually an appetite.
Related: Proverbs 19:5 · Proverbs 19:9 · Proverbs 13:23 · Proverbs 21:28
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.