zwieback

from WordNet (r) 3.0 (2006)

zwieback
    n 1: slice of sweet raised bread baked again until it is brown
         and hard and crisp [syn: {zwieback}, {rusk}, {Brussels
         biscuit}, {twice-baked bread}]

from The Collaborative International Dictionary of English v.0.48

Zwieback \Zwie"back`\ (tsv[=e]"b[aum]k`), 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.
   [Webster 1913 Suppl.]