Private eye- investigator, vocabulary
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The term private eye alludes simply to private i (short for an investigator).

You may also call such a person a tec (short for detective), a gumshoe (from a quiet, rubber-soled footwear), a sleuth (from an Old Norse word for “trail”), a shamus (of Yiddish origin), or a hawkshaw (from a detective in the 1863 play The Ticket of Leave Man).