Where does the word “hardtack” come from and What does hardtack mean?

If the term “hardtack” were not already available it is possible that today’s mariner would invent a term of similar import, such as “hard grub,” as all that was intended by tack, a hundred years ago, was food of any sort.

Hard tack was literally hard food, or, that is, biscuit of more than ordinary hardness to have on hand in rough weather.

The modern slang equivalent is dog biscuit.