Green’s Dictionary of Slang

Johnnie Nab n.

[johnny n.2 + nab v.1 (3)]

(US black) a police officer, usu. male.

[US]Drake & Cayton Black Metropolis 611: Then (as one ‘tapper’ expressed it), ‘Johnnie Nab comes through and takes every “cat” and his brother to California’ (i.e., to the jail on California Avenue).