blue belly
/ˈbluː ˌbɛlɪ/originally and chiefly United States regional (southern ). Usually derogatory
noun
A blue-bellied person, one regarded as low, contemptible, or cowardly; specifically (a) a northerner; (b) a northern soldier, especially during the American Civil War (1861–5).
Origin
Early 19th century; earliest use found in John Pickering (1777–1846). From blue + belly, after blue-bellied.