A fish with a grey back, or in historical contexts, a Confederate soldier (from the grey-colored uniform).
Compound word from 'grey' plus 'back,' descriptive of the coloring. During the American Civil War, it gained slang usage referring to Confederate troops who wore grey uniforms.
The nickname 'greyback' for Confederate soldiers shows how descriptive compound words emerged as battlefield slang, and these wartime terms sometimes entered dictionaries because they appeared so frequently in Civil War literature and letters.
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