Modern crackers are the end product of what started as a convenience food for soldiers and sailors, recorded as far back as 1190. Known as hard tack or ship's biscuit, they were made out of a mixture of grain and/or legume flour and water, shaped into tablets, then baked until brick hard.
https://en.wikipedia.org/wiki/Cracker_(food)

Hardtack (or hard tack) is a type of dense cracker made from flour, water, and sometimes salt. Hardtack is very inexpensive and long-lasting, allowing it to be used for sustenance in the absence of perish.
https://en.wikipedia.org/wiki/Hardtack