n. a police officer or a detective. (Probably from Yiddish.) : One thing that a shamus always knows is when another shamus is following him.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
shamus
"police officer, detective," 1925, probably from Yiddish, lit. "sexton of a synagogue," from Heb. shamash "servant;" influenced by Celt. Seamus "James," as a typical name for an Irish cop.