A privately employed detective, as opposed to one working for the police or another authority. For example, The children loved stories about private eyes, and Janey wanted to become one. This expression comes from the term private investigator, the "i" of investigator being changed to "eye," which plays on the idea of a person looking into things. [1930s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.