Green’s Dictionary of Slang

a generic (and thus derog.) name for any African-American man; orig. given to slaves.

1811		N.Y. Morning Post 3 Apr. 2/4: An honest Hibernian standing hard by, [...] gave poor Cato such a dig in his munns, that he was no longer able to tell what he wanted.
1833		Albany Microscope (NY) 28 Dec. n.p.: ‘Don’t you open your mouf, Cato till I’m done [...] you too, Pompey’.
1847		Nat. Police Gaz. (NY) 14 Aug. 389/4: free negroes [...] Cato Rickets, seaman.
1851		Manchester Spy (NH) 17 May n.p.: ‘Gib me de defintion ob de big word belle-letters? Sartin, Cato — I won’t do nuffin else. De fac am, you unlettered colored man [etc].
1875		F.H. Sheppard Love Afloat 271: He had occasion to go to New York, and like a darn fool – beg your pardon, ladies – he took Cato along with him.