The Collaborative International Dictionary of English v.0.48:

Digamma \Di*gam"ma\, n. [Gr. ?; ? = ? twice + ga`mma the letter
   [Gamma]. So called because it resembled two gammas placed one
   above the other.] (Gr. Gram.)
   A letter (?, ?) of the Greek alphabet, which early fell into
   disuse.
   [1913 Webster]

   Note: This form identifies it with the Latin F, though in
         sound it is said to have been nearer V. It was
         pronounced, probably, much like the English W.
         Digammate