shorthand for "private investigator" (private "i".. get it?). interestingly enough, the pinkerton national detective agency, the first detective agency in the united states, had as its slogan, "we never sleep". they had, as their logo, a big ole eye, symbolizing their vigilance and ever-watchfulness.

the pinkerton national detective agency was founded by allan pinkerton in 1850. encyclopedia.com has this to say about him:

Pinkerton, Allan
1819-84, American detective; b. Scotland. In Chicago he founded (1850) what became the Pinkerton National Detective Agency. During the Civil War he directed an espionage system behind the Confederate lines. His agency, which solved numerous railroad robberies, also gathered the evidence that broke up the MOLLY MAGUIRES in the 1870s. It subsequently became notorious as a private police force for hire to management as strikebreakers.

this company is still in existence - "Pinkertons, Inc. is the World Class Enterprise Security Solution for the 21st Century". visit them at www.pinkertons.com.

they still have an eye logo.