Green’s Dictionary of Slang

1. (US, Southern) a Northerner, a Yankee, esp. a Northern soldier during the Civil War (1861–5) [the uniform of the Northern troops although note earlier blue-bellied adj.].

1857		T.H. Gladstone Englishman in Kansas 43: No highfalutin’ airs here, you know. Keep that for them Yankee Blue-bellies down East.
1863		letter in B.I. Wiley Life of Johnny Reb (1943) 314: You damn cowardly Scoundrels [you can’t] face the music you blue bellys.
1872		Schele De Vere Americanisms 280: The Northerners were Yanks, or Bluebellies (from their blue uniform), or Boys in Blue.
1883		Daily Tel. 9 Feb. 5/4: The Confederate armies during the great Civil War in America... were known as ‘Greybacks,’ whereas their Federal opponents, from the light-azure gaberdines which they wore, were dubbed ‘blue-bellies.’ [F&H].
1894		(con. 1861–5) G.A. Sala Things I Have Seen I 237: The South [...] called the Federal soldiers, whose upper garment was a kind of gabardine of a light azure hue, ‘filthy bluebellies.’.
1944		(ref. to Civil War) J.S. Pennell Hist. of Rome Hanks 180: The bluebelly’s legs drew up a little [...] Lacey was taking off the Yankee’s shoes.

2. a police officer; thus blue-bellied, a derog. adj. for a member of the police [the uniform].

1866		H.L. Williams N.-Y. After Dark 77: ‘Peelers clubbing a man for cheering our man!’ shouts another. ‘Rescue, rescue fellers!’ yells a third. ‘Soak the ---- bluebellies! remember the Riot week! Close in, close in, an’ they can’t use their clubs!’.
1882		‘The Jargon of Thieves’ in Derry Jrnl 8 Sept. 6/6: If a hoodlum should remark , ‘I gave the bluebelly a fill,’ he would [...] mean that he had succeeded in deceiving a police officer.
1909		J. Ware Passing Eng. of the Victorian Era 130/2: Fill, To give a (Thieves’). To deceive, e.g., ‘I gave the blue belly a fill’ – would mean that you sent the policeman on a wrong scent.
1923		Kipling ‘The Janeites’ in Debits and Credits (1926) 158: One o’ them blue-bellied Bolshies of postwar Police (neglectin’ point-duty, as usual) asked us to flirt a little quieter.
1949		Monteleone Criminal Sl. (rev. edn).
1972		B. Rodgers Queens’ Vernacular.
2001		N. Green Shooting Dr. Jack (2002) 249: That buildin’s a bad place for you and Tuco. A few of them bluebellies get curious and go lookin’, you got no excuse for bein’ there.