Green’s Dictionary of Slang

(US) an undercover agent.

1986		T. Philbin Under Cover 53: They burned a UC about three months ago. He infiltrated. But they got him.
2006		(con. 1972) Jurgenson & Cea Circle of Six 89: None of the detectives [...] read prototypical cop. They were excellent UCs, all with a lot of time spent in overt surveillance.
2019		D. Winslow Border [ebook] ‘I think you’re a snitch. I think you’re UC, and I think the only question is whether we shoot you or throw you off the roof’.