It's a Jewish thing?!

Nimoy drew on his orthodox Jewish upbringing to invent the iconic hand gesture, and he wrote about the process of finding it in his memoir, I am Spock:

For what would soon become known as the Vulcan salute, I borrowed a hand symbol from Orthodox Judaism. During the High Holiday services, the Kohanim (who are the priests) bless those in attendance. As they do, they extend the palms of both hands over the congregation, with thumbs outstretched and the middle and ring fingers parted so that each hand forms two vees. This gesture symbolizes the Hebrew letter shin, the first letter in the word Shaddai, `Lord.’ … So it was that, when I searched my imagination for an appropriate gesture to represent the peace-loving Vulcans, the Kohanim’s symbol of blessing came to mind.

In a chat with the Baltimore Sun in 2000, Nimoy explained how he got the idea:

In the blessing, the Kohanim (a high priest of a Hebrew tribe) makes the gesture with both hands, and it struck me as a very magical and mystical moment. I taught myself how to do it without even knowing what it meant, and later I inserted it into “Star Trek.” There was a scene in one episode that needed something. People were seeing other members of the Vulcan race for the first time, and I thought it called for a special gesture.