The Collaborative International Dictionary of English v.0.48:

Rusk \Rusk\, n. [Sp. rosca de mar sea rusks, a kind of biscuit,
   rosca properly meaning, a screw, spiral.]
   1. A kind of light, soft bread made with yeast and eggs,
      often toasted or crisped in an oven; or, a kind of
      sweetened biscuit.
      [1913 Webster]

   2. A kind of light, hard cake or bread, as for stores.
      --Smart.
      [1913 Webster]

   3. Bread or cake which has been made brown and crisp, and
      afterwards grated, or pulverized in a mortar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rusk
    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]