hardtack is a compound of hard and tack, the latter a nineteenth-century sailors' slang term for food or fare; the biscuit's notorious toughness gave it the name. Morphologically, the word is best read through the components named in the history, with the modern spelling preserving a compound structure rather than a single unanalyzable stem. In present English, the noun sense is "a hard biscuit made of flour and water, used as rations at sea or on campaign", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hard and tack. Its semantic neighborhood includes biscuit, sea-biscuit, and pilot-bread, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English heard.