Quick answer
Send a sluggard on an errand and you will feel it the way teeth feel vinegar and eyes feel smoke: sharp, maddening, and entirely predictable.
Read both lines
The humor is the teaching method. Both images are small, immediate miseries: vinegar sets the teeth on edge, smoke stings and blinds, and neither kills you while consuming your whole attention. That is the sender waiting on a lazy messenger: the errand not done, the answer not come, the irritation total. Notice whose experience the proverb takes: not the sluggard's but the sender's. Laziness likes to think of itself as a private, victimless pace. The proverb disagrees: somebody sent you, and somebody is squinting through smoke waiting.
Related: Proverbs 25:13 (the faithful messenger · this verse's happy twin) · Proverbs 26:13–16
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.