"Like vinegar to the teeth and smoke to the eyes, so is the lazy one to those who send him"
vinegar to the teeth
Vinegar wine – which the grape-growing Israelites would be familiar with – has an especially irritating flavor and sting when applied to the teeth or inside the mouth.
smoke to the eyes
Smoke – when it is blown in the eyes – is particularly irritating to these organs.
In just this way Solomon is saying that the person who is lazy will be particularly irritating to the person who is trying to get work done through the lazy person. It will be a major stinging and irritating aspect of work. Don't do this. No matter if the person needs a job, is a relative, promises that they will do better than the last time. Do not use a lazy person. Do not count on them to do a particular thing. It will not happen that way.
This is the Hebrew word atsel which means sluggish, lazy. This is the person who does not want to work. This is the person who is slow in his/her work. This is the person who does not do portions of his/her work.
Solomon is trying to point out what happens to those who try and do a good deed by hiring the lazy person. He says it is like putting vinegar on your teeth or having smoke blown in your eyes. This is one of those leadership-lessons proverbs. Solomon is saying that as a leader you cannot afford to hire or use the lazy person because you will be the person who pays.