Green’s Dictionary of Slang

(US) uncle.

1884		G.W. Bagby Old Virginia Gentleman (1910) 69: ‘Unc’ Jim’ – as he is generally called, for short.
1903		‘Hugh McHugh’ Out for the Coin 28: I never did Unc any harm in life.
1937		J.T. Farrell ‘All Things are Nothing to Me’ in Fellow Countrymen 61: Yes, Unc. The grass will be green.
1962		B. Veeck Veeck — as in Wreck 139: [H]e called back to tell me that Uncle Clark had agreed. I wasn’t going to give Unc a chance to have any second thoughts.
1970		B. Jackson Get Your Ass in the Water (1974) 58: I see your lips quiver, Unc, but I don’t hear a cocksucken word you sayin’.
1985		J. Sullivan ‘As One Door Closes’ Only Fools and Horses [TV script] I never thought of that Unc!
1986		L. Heinemann Paco’s Story (1987) 202: Wounded in the war, Unc says.