It entered the Irish and Scottish Gaelic languages from the French variation of the late Latin name for Jacob, Iacomus; a dialect variant of Iacobus, from the New Testament Greek Ἰάκωβος (Iákōvos), and ultimately from Hebrew word יעקב (Yaʻaqov), i.e. Jacob.
The name comes either from the Hebrew root עקב ʿqb meaning "to follow, to be behind" but also "to supplant, circumvent, assail, overreach", or from the word for "heel", עֲקֵב ʿaqeb.
[2][3] The traditional explanation for the name follows that it was given to the patriarch Jacob when he was born, as he was grasping his twin brother Esau's heel, though this is a folk etymology.
In the United States, the word "Shamus" is a misspelling of Séamus and was a derogatory slang for a persisting stereotype of an Irish-American police officer — especially detectives and private investigators — continuing through to today, but at a much lesser degree.
The great preponderance of Irish immigrants found employment in the police departments, fire departments, and other public services of the major cities, largely in the northeast of the US and around the Great Lakes, and have been considerably over-represented in the New York police since then.