Green’s Dictionary of Slang

a detective.

1866		in Nat. Police Gaz. (NY) 29 Dec. 2: Detective ‘Hawkshaw’ [...] says he is going to do the same thing that the ‘detecs’ do up above .
1884		Capt. — Eighteen Months Imprisonment in DSUE (1984).
1885		G.M. Baker Globe Drama 49: Got de bead on de detect. Now’s yer chance: hoof it.
1928		M.C. Sharpe Chicago May (1929) 234: I hated to leave the hospital, when the detects came for me.