Origin: Ger < zwie-, two, twice, var. of zwei, two + backen, to bake, calque of It biscotto, biscuit
See zwieback in American Heritage Dictionary 4
Origin: : zwie-, twice (from Middle High German zwi-, from Old High German; see dwo- in Indo-European roots)
Origin: + backen, to bake (from Middle High German, from Old High German bahhan, backan).