It's definitely not slang since it goes to back to Roman times. It's believed that the origin has to do with a remedy for poison that included a small amount of salt.

It's an idiom. And as another commenter previously wrote, it goes back to Roman times. If you're interested, here's a short article about the idiom. But take it with a grain of salt! =P https://www.mentalfloss.com/article/648536/take-it-grain-salt-meaning-and-origins