Shamus

google

ref

1920s: of unknown origin.

wiktionary

ref

Said to be from the Irish name Séamus, on account of many American police officers being of Irish descent.

etymonline

ref

shamus (n.)

"police officer, detective," 1920, apparently first in "The Shamus," a detective story published that year by Harry J. Loose (1880-1943), a Chicago police detective and crime writer; the book was marketed as "a true tale of thiefdom and an expose of the real system in crime." The word is said to be probably from Yiddish shames, literally "sexton of a synagogue" ("a potent personage only next in influence to the President" [Israel Zangwill]), from Hebrew shamash "servant;" influenced by Celtic Seamus "James," as a typical name for an Irish cop.