Green’s Dictionary of Slang

a plain-clothes detective or undercover agent.

1931		Irwin Amer. Tramp and Und. Sl. 193: under cover man. – An employer’s detective or spotter watching the workers without their knowledge.
1949		Monteleone Criminal Sl. (rev. edn).
1972		J. Mills Report to the Commissioner 77: ‘[I]f we bust the three of them, how are we gonna cut the undercover loose without it lookin’ bad, right?’.
1996		(con. mid-1960s) J. Lardner Crusader 308: When finance department undercovers made a new round of visits [...] they were turned down.
2005		R. Cea No Lights, No Sirens 104: They [...] would have to be proficient in telling the difference between the real junkies and the undercovers looking to jack the spot .
2020		L. Lungaro The 3-0 16: Detective Christopher Hoban [...] was a caring, humble undercover who gave so much to this city.