The metal hand sign is from an Italian gesture to ward off the evil eye.
Ronnie James Dio, considered one of the greatest metal vocalists, fronted a host of legendary bands – Rainbow, Dio, and Black Sabbath. In 1979, when he was replacing Ozzy Osbourne in Sabbath, he wanted to set himself apart from his predecessor. Ozzy would flash the peace sign during concerts and Dio wanted to do something different. He thought back to the hand gesture his Italian grandmother would use to ward off the “malocchio” or “the evil eye.” From this, Dio began to flash the “mano cornuto” or the “horned hand” gesture.
The mano cornuto is related to the Italian cornicello charm of a single horn that looks a bit like a chili pepper. They’re both used as protection against the evil eye. The hand gesture has the index and pinky fingers extended and may have evolved from the idea of the two extended fingers “poking the eyes” of the person giving you the evil eye.
Because Dio was using the sign as the vocalist for Black Sabbath, he helped popularize the gesture in heavy metal culture. Soon other musicians, as well as fans, began to make the same gesture and today it’s used all over pop culture. Dio never claimed to have invented the sign but he certainly did more to make it a part of heavy metal than anyone else.
Added info: Gene Simmons of KISS, never one to pass up an opportunity to shamelessly profit off of something, filed an application to trademark a strikingly similar hand sign. In 2017 he tried to trademark the metal hand sign but with the thumb extended instead of tucked in. What Simmons claimed was his also happens to be the sign for “I love you” in American Sign Language. He later withdrew his application but not before Dio’s widow, Wendy Dio, said of Simmons “To try to make money off of something like this is disgusting. It belongs to everyone; it doesn’t belong to anyone. It’s a public domain; it shouldn’t be trademarked.”