1. What does it mean?
An abbreviation of 갑자기 분위기가 싸해짐: the atmosphere suddenly turns chilly. It describes a conversation or gathering that abruptly becomes uncomfortable, often after an ill-timed remark or a joke that does not land.
2. The nuance
The chill is social, not about temperature. Calling a moment 갑분싸 can itself embarrass the person who caused it, so it works better as a light retelling than as a way to call someone out.
3. Examples
“내 농담에 아무도 안 웃어서 갑분싸 됐어.”
Nobody laughed at my joke, and the room went awkwardly quiet.
“잘 놀다가 갑자기 시험 얘기를 꺼내서 갑분싸였어.”
We were having fun, then someone brought up exams and killed the mood.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.