Green’s Dictionary of Slang

a Private I, i.e. eye.

1953		R. Chandler Long Good-Bye 10: ‘Oh ho,’ he said. ‘A P.I. picking up a client.’.
1986		C. Hiaasen Tourist Season (1987) 30: Keyes had been a solid reporter and it was a damn shame he’d given it up to become a P.I. Joe Allen wasn’t crazy about private investigators.
1990		S. Geason Shaved Fish 154: ‘Who are you?’ he said finally. [...] ‘I’m a PI.’.
1999		Guardian G2 10 Nov. 8: The cops were always dragging the PI into the cop shop.
2004		J. Ellroy ‘Jungletown Jihad’ in Destination: Morgue! (2004) 366: I figured you was a PI, then I saw your badge.
2010		L. Redhead Thrill City [ebook] I wanted to introduce a female PI in my next Zack book.
2013		T. Robinson Hard Bounce [ebook] ‘There’s gotta be two hundred PIs in Boston’.
2021		J. Ellroy Widespread Panic 32: I promised him two g’s for a PI’s ducat, quicksville.