From 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]