I'm not sure about you but I have heard this phrase many times through out my lifetime and I have to admit the very first time I heard it as a kid I had no clue as to what it meant. Me being the observant and quiet person I kept my mouth shut and decided to figure it out later. When I was a kid the Internet was not as readily available as it is today, so I couldn't just go home and look it up on Google. It was probably not until I heard it a few times in the movies or heard a friend or two say it that I finally caught the meaning. Ah ha! To be worth one's pay or to be effective, but where does the salt come in?
Apparently salt was a pretty big deal before refrigerators were invented because it was used to help preserve food. In addition to preserving food, salt was also an important commodity. In the Roman days salt was used as an item of trade and for its health benefits. Back then soldiers were given salarium which was an allowance for purchasing salt. The sal in salarium means pay in Latin and over time the word was shorted to salary. So because the words salarium and salt went hand in hand, the word salt became associated with pay or worth.
Aren't you glad I cleared that up for you? Don't you feel more knowledgeable?
Good. Now get out there and prove your worth in salt to the world!