1. What does it mean?
먹금 shortens 먹이 금지 or 먹이 제공 금지. The 'food' is engagement: replies and outrage can give an attention-seeking post exactly what it wants.
Also written or said as 먹이 금지, meokgeum
2. The nuance
It means withholding engagement, not necessarily ignoring every problem. Reporting abuse or helping a target is different from joining a baited argument.
3. Examples
“계속 같은 말로 싸움 거는 계정은 먹금하자.”
Let's stop engaging with the account that keeps repeating the same provocation.
“먹금한다고 피해 신고까지 안 하는 건 아니야.”
Not feeding the troll doesn't mean we can't report the harm.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.