zwieback
Zwieback (noun) means a form of rusk, usually sweetened bread enriched with eggs that is baked and then sliced and toasted until dry and crisp; considered easy to digest and therefore given to the ill and used as a teething food for toddlers.
Etymology
Comes from German Zwieback, made up of zwie- ("twi-, two-") and backen ("to bake"), so literally "twice-baked". It is a calque of Italian biscotto, though not to be confused with that word.