Green’s Dictionary of Slang

(US) Colt Model 1861 Navy cap & ball .36-caliber revolver (a six-shot, single-action percussion weapon produced by Colt's Manufacturing Company 1861-1873).

1878

Nat. Police Gaz. (NY) 27 Apr. 6/4: Dave drew his navy, and he laid that navy and Clay down together [...] Clay was as lifeless as the instrument which had chilled the circulation of his body.