A person who informs on someone else, especially to the police. The gang was wary of him, suspecting he might be a nark.
A police officer or government agent who enforces drug laws. The undercover nark had been gathering evidence for months.
A person who habitually complains, spoils other people’s enjoyment, or acts as a killjoy (chiefly Australian & New Zealand, informal). Don’t be such a nark—come out and enjoy yourself.