Where did the origin of throwing salt over your back after you spill some come from?

Superstitions about salt date back to biblical times when salt was a highly prized commodity. It was expensive, crucial in preserving food, and was often used in lieu of currency. So spilling salt was considered an almost sacrilegious offence, and left one perilously exposed to the devil’s machinations.

Throwing salt over your shoulder is akin to blessing someone after they’ve sneezed—it’s a way of keeping the devil at bay while you’re in an especially vulnerable moment. Depending on your interpretation, the salt is either intended to blind the devil so he can’t witness your error, or keep him from sneaking up on you while you’re cleaning up your mess.

I found this with Google

Throwing salt over your left shoulder is how it is supposed to be done. It was the left shoulder because the devil is supposed to be over your left shoulder. The salt blinded the devil. By the way. It would be an old Wives tale. :)