Green’s Dictionary of Slang

(US Und.) pertaining to the Pinkerton Detective Agency.

1900		Flynt & Walton Powers That Prey 32: You coppers got to help him. I ain’t going to have the Eye people snake in all the loose coin; I give it to you straight.
1940		D. Maurer Big Con 148: The jug where his cush is, is an eye gaff.