hardtack       
noun [ U ] uk/ˈhɑːd.tæk/ us/ˈhɑːrd.tæk/
 压缩饼干
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。 
 - More examples
 - The cook added some salt into the hardtack to make it a bit more palatable.
 - It's not known when people first started making hardtack, but it was certainly many centuries ago.