Quick answer
What goes on inside shows up physically. The verse names envy specifically as the thing that eats a person from within.
Read both lines
The claim is psychosomatic in the plainest sense: a settled interior is life to the flesh, and a corroded one gets into the bones. Envy is a well-chosen example because it is entirely internal, requires no action, harms nobody else directly, and still does damage to its owner. The image of rotting bones was used at 12:4 of a shaming marriage, so this book reserves it for what degrades a person structurally rather than superficially. Nothing here suggests illness is caused by sin. It observes that some interior states cost the body something.
Related: Proverbs 12:4 · Proverbs 17:22 · Proverbs 4:23
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.