Narc as in short for Narcissistic Personality Disorder.
There is a lot of debate about personality disorders within mental health and how they should be categorised anyway.
Often this term is used on Mumsnet when the person has never been diagnosed with any form of mental health issue, it's just a lazy shorthand for behaviours that the poster doesn't like. Then you have people who've never even met the person being discussed diagnosing them with it on here.
AIBU to think that armchair diagnosis is a bad idea and it needs to stop?