1. What does it mean?
ㄴㄴ shortens 노노, the Korean spelling of 'no no.' It can reject a suggestion, correct a misunderstanding, or tell a friend that something is unnecessary.
Also written or said as 노노, ㄴㄴㄴ, nono
2. The nuance
The short form may sound abrupt without context. For an important refusal, explain in a full sentence rather than relying on two letters. Repetition is a writing style, not a separate level of politeness.
3. Examples
“오늘 도서관 쉬는 날이야? ㄴㄴ, 여섯 시까지 열어.”
Is the library closed today? Nope, it's open until six.
“ㄴㄴ 우산 안 가져와도 돼. 여분 있어.”
No, you don't need to bring an umbrella. I have a spare.
Sources
FluentU: Korean text slang guide
References support the meaning and usage. Example sentences are written for NoCapWiki.