a private eye
slang A private investigator or detective. "Eye" is a pun on the letter "i" in "investigator." The disgraced police detective moved to another state and started a business as a private eye. It turned out they'd hired a private eye to keep tabs on her after she left the law firm.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a ˌprivate ˈeye(informal) a detective who is not in the police, but who can be employed to find out information, find a missing person, follow somebody, etc: They hired a private eye to look for more evidence.
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