Lewis & Short

dĭgammon, i, or dĭgamma, ătis, n., also dĭgammos, i, f. (sc. littera), = δίγαμμον (sc. στοιχεῖον) or δίγαμμα,

  1. I. the Aeolic double gamma or digamma (ϝ), represented in Latin sometimes by V, sometimes by F, which, written upside down, thus, F, the Emperor Claudius wished to substitute for both F and V, Quint. 1, 7, 27; cf. Tac. A. 11, 14; Suet. Claud. 41 (v. the letters F and V).
          1. (α) Digammon, Quint. 1, 4, 7 Zumpt and Meyer N. cr.; Prob. Verg. G. 1, 70; Don. p. 1736 P. Lind. N. cr.; Cassiod. p. 2292 P.
          2. (β) Digammos littera, Ter. Maur. p. 2387 P.; and simply digammos, Serv. Aen. 1, 292; 642; 6, 359; Pompei. ad Don. p. 21 Lind. N. cr.
          3. (γ) Digamma, Prisc. p. 542 P.; 545 fin. ib.; 709 ib. al.
  2. II. Used jestingly to denote an incomebook (from the title Fundorum reditus, the first letter of which is a digamma), Cic. Att. 9, 9, 4.