slang A private detective or police officer. Possibly derived from the Yiddish term shammes (a synagogue attendant) or from the Irish name Seamus (alluding to the prevalence of Irish-Americans in US police forces, especially in the early 20th century). He put his military training to good use as a shamus for hire, tracking down errant spouses and runaway debtors. There's been some shamus snooping around our activities in the docks, boss? You want us to send him to sleep with the fishes?
n. a police officer or a detective. (Probably from Yiddish.) One thing that a shamus always knows is when another shamus is following him.