The Latin word for salt is sal. Salt was an important commodity in the Roman empire. It was used to flavour food and more importantly to preserve food at a time when refrigeration did not exist. Roman soldiers were given regular rations of precious salt or they were given an allowance to buy salt. That allowance was called a salarium. And from salarium, we derive our English word salary.
Now, is that where we get the expression, "a worker who is worth his salt"? A quick search reveals that the phrase "worth is salt" goes only as far back as 1830. One appearance was in Robert Louis Stevenson's Treasure Island: "It was plain from every line of his body that our new hand was worth his salt."
So is the phrase "worth his salt" related to being paid in salt? I have no idea though it is enticing to link the two.
