The Collaborative International Dictionary of English v.0.48:

Quaff \Quaff\, v. t. [imp. & p. p. Quaffed; p. pr. & vb. n.
   Quaffing.] [For quach, fr. Gael. & Ir. cuach a drinking
   cup; cf. L. caucus a drinking vessel. Cf. Quaigh.]
   To drink with relish; to drink copiously of; to swallow in
   large draughts. "Quaffed off the muscadel." --Shak.
   [1913 Webster]

         They eat, they drink, and in communion sweet
         Quaff immortality and joy.               --Milton.
   [1913 Webster]