Also called ship biscuit and sea bread, this large, hard biscuit is made with an unsalted, unleavened flour-and-water dough. After it's baked, hardtack is dried to lengthen shelf life. It's been used at least since the 1800s as a staple for sailors on long voyages.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.