The Collaborative International Dictionary of English v.0.48:

Sack \Sack\ (s[a^]k), n. [OE. seck, F. sec dry (cf. Sp. seco,
   It. secco), from L. siccus dry, harsh; perhaps akin to Gr.
   'ischno`s, Skr. sikata sand, Ir. sesc dry, W. hysp. Cf.
   Desiccate.]
   A name formerly given to various dry Spanish wines. "Sherris
   sack." --Shak.
   [1913 Webster]

   Sack posset, a posset made of sack, and some other
      ingredients.
      [1913 Webster]