Definition of blue belly in English:

blue belly

Pronunciation /ˈbluː ˌbɛlɪ/

noun

dialect, derogatory US
  • 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.