A narc is an officer who deals with illegal drugs. An undercover narc, spotted the narc, working as a narc.
It is American slang and unfriendly, since it is used by the people being investigated. As a verb it has widened well beyond drugs: to narc on somebody is to inform on them about anything, and it is playground language as much as street language.
1nounnarcAn officer who deals with illegal drugs.
“The dealers suspected the new guy was a narc working for the police.”
narc is an officer.
A clipping of narcotics, from Greek narkotikos, numbing, from narke, numbness.
a clipping of narcotics.
Etymology from Wiktionary, CC BY-SA