digamma n.

1550s, "the letter F;" 1690s as the name of a former letter in the Greek alphabet, corresponding to -F- (apparently originally pronounced with the force of English consonantal -w-), from Latin digamma "F," from Greek digamma, literally "double gamma" (because it resembles two gammas, one atop the other). The sixth letter of the original Greek alphabet, it corresponded to Semitic waw.