Uncle Nabs, n. — Green’s Dictionary of Slang

Uncle Nabs n.

also Betty Nabs, Mr Nabs

(US black) the police.

1942		D. Burley N.Y. Amsterdam Star-News 21 Mar. 16: ‘In brooms a Mr Nabs and lays his gims on me ’.
1942		D. Burley N.Y. Amsterdam Star-News 9 May 12: ‘Betty Nab,’ the police.
1980		E. Folb Runnin’ Down Some Lines 67: Characteristic of the police-related lexicon is an ironic, if sometimes grim, humor that is embodied in names like [...] Uncle Nab.