In re: etymology of the word. There are websites for this:
1967 (earlier narco, 1960), American English slang, shortened form of narcotics agent. It had been used 1955 for narcotics hospital, 1958 for narcotics addict. The senses and spelling have tended to merge with older but unrelated nark (q.v.).
1859, “to act as a police informer” (v.); 1860, “police informer” (n.), probably from Romany nak “nose,” from Hindi nak, from Sanskrit nakra, which probably is related to Sanskrit nasa “nose” (from PIE root *nas- “nose”). Sense and spelling tending to merge with etymologically unrelated narc (q.v.).