What is the origin of spilling salt superstition?

Lots of superstitious people, when they spill salt, will take a pinch of it in their right hand and throw it backwards over their left shoulder.

Superstition has it that, at all times, a devil waits over your left shoulder, and also that spilling salt is bad luck (perhaps because it used to be rare and precious). Hence, you can mitigate your bad luck by throwing salt into the eyes of the devil. There's also a belief that your guardian angel (who can be found over your right shoulder) spills salt to warn you of evil nearby. Either you throw the salt to hurt the devil or, as salt was valuable, as an offering to placate him.

Like lots of other superstitions in a Christianized culture, it has a Christianized explanation: spilling salt is bad luck, because Judas spilled salt at the Last Supper. This is somewhat equivalent to the Christianized explanation for not sitting down thirteen at the table, and almost certainly a red herring.