Why is spilling salt considered bad luck?

The European belief in the ill luck that comes from spilt salt is quite old, going back to ancient Rome. The 1556 "Hieroglyphica" of Piero Valeriano Bolzani reports that salt was formerly a symbol of friendship, because of its lasting quality. It was usually presented to guests before other food, to signify the abiding strength of friendship.

Some say the root of this superstition is that salt was a precious commodity in ancient societies. Because of its difficulty to procure and its high cost, salt became a form of currency. In fact, the word "salary" originates from sal, the Latin word for salt, possibly because Roman soldiers received salt as part of their compensation.There is no historical evidence for this belief.

Spilling something as highly prized as salt was bad form and a big waste, which grew into a warning that carelessness would bring bad luck. As superstitions tend to do, the spilled salt superstition grew and evolved over centuries, with different cultures assigning different meanings.

According to another legend, spilling salt invites the devil, who supposedly waits behind your left shoulder, to perform evil deeds.

Some also say it's because of Leonardo da Vinci's painting "The Last Supper." In the painting, one can see some spilled salt near Judas Iscariot's elbow, which he presumably knocked over. According to the Bible, Judas later betrays Jesus Christ, so spilling salt became associated with dishonesty and treachery bringing bad luck.