Source : Webster's Revised Unabridged Dictionary (1913)

Zwieback \Zwie"back`\, n. [G., fr. zwie- two, twice (see
   {Twice}) + backen to bake.]
   A kind of biscuit or rusk first baked in a loaf and
   afterwards cut and toasted.

Source : WordNet®

zwieback
     n : slice of sweet raised bread baked again until it is brown
         and hard and crisp [syn: {rusk}, {Brussels biscuit}, {twice-baked
         bread}]