A private detective or investigator, especially in noir detective fiction and slang.
From Yiddish 'shames' or 'shammes' (a synagogue caretaker or sexton), borrowed from Hebrew 'shamash.' Somehow the Yiddish term became slang for private detective, possibly from association with a person who knew everyone's secrets.
The word 'shamus' perfectly captures the immigrant influence on American English—a Hebrew-Yiddish word for a synagogue official somehow transformed into hardboiled detective slang, showing how immigrant communities shaped American pop culture and language.
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