Thousands of years ago, before money was introduced, workers and soldiers were often paid with a negotiated quantity of salt.
More than as a seasoning, salt’s value was in its use as a preservative or cure for meat, as well as a medicine.
The early Romans called this payment a “salarium,” which gave us the word salary.
If a man wasn’t worth his salt, he wasn’t worth his salary, or in other words, he isn’t up to the job.