1US historical Any of several wading birds; especially the (red) knot, Calidris canutus, in winter plumage.
2British, US dialect, rare The (greater) scaup, Aythya marila.
3US dialect, archaic, historical A louse. Also figurative.
4US More fully "greyback whale". The grey whale, Eschrichtius robustus.
5British dialect, rare More fully "greyback crow". The hooded crow, Corvus corone cornix. Compare grey crow .
6archaic, historical A member of a military or police force whose uniform includes, or consists of, a grey coat; specifically (US, informal) a soldier of the Confederate army in the American Civil War (1861–5). Compare greycoat , "grey".
7US historical A monetary note issued by the Confederacy during the American Civil War. Compare "blueback", "greenback".
Early 19th century; earliest use found in Alexander Wilson (1766–1813), ornithologist and poet. From grey + back.