a kind of biscuit made from flour and water and previously eaten especially by sailors and soldiers

Hardtack was ideal for long sea voyages as it would remain edible for a very long time.

Hardtack is so dry and hard that you really need to dip it in something or soften it in a pan with some fat. 压缩饼干又干又硬，需要蘸着吃，或者在锅里放油把它弄软。