1. What does it mean?
눈새 abbreviates 눈치 없는 새끼, using a coarse insult to describe someone seen as failing to read the room. It may target an ill-timed comment or a failure to notice another person's discomfort.
Also written or said as nunsae
2. The nuance
It is harsher than simply saying someone misunderstood. Missing an unspoken cue is not a character defect; stating what you need is clearer than insulting the person.
3. Examples
“말을 끊은 친구에게 눈새라고 했다가 표현이 심했다고 사과했어.”
I called a friend clueless for interrupting, then apologized for the harsh wording.
“처음 온 사람을 눈새라고 하지 말고 상황을 설명해 줘.”
Explain the situation to the newcomer instead of insulting them for missing the cues.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.