Part of the quarrels run, and the first of three verses on the same figure.
Quick answer
Hatred talks pleasantly. The verse says the disguise is on the lips and the actual content is being stored somewhere else.
Read both lines
Two locations and that is the argument: the lips do the disguising, the inside does the storing. The verb behind "lays up" describes setting something down deliberately, so this is not a man losing his temper and hiding it afterward. He is accumulating.
The received text carries a variant on whether the disguise is on his lip or his lips, which changes nothing. What makes the verse useful rather than merely cynical is that it explains the previous verse's mechanism: the silver dross had to be applied by someone, and this is him applying it.
Related: Proverbs 26:23 · Proverbs 10:18 · Proverbs 26:25 · Proverbs 27:6
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.