According to a Wikipedia entry, “the hamesh hand or hamsa hand is a popular motif in Jewish jewelry. Go into any Jewish gift shop and you will find necklaces and bracelets bearing this inverted hand with thumb and pinky pointing outward. The design commonly has an eye in the center of the hand or various Jewish letters in the middle.”
“There is nothing exclusively Jewish about the hamesh hand. Arab cultures often refer to it as the Hand of Fatima, which represents the Hand of G-d. Similar designs are common in many cultures. Why it has become such a popular symbol among Jews? I haven’t been able to find an adequate explanation anywhere. My best guess: in many cultures, this hand pattern represents a protection against the evil eye, and the evil eye has historically been a popular superstition among Jews.
