1. What does it mean?
심멎 compresses the idea of 심장이 멎다, the heart stopping. In fan comments and casual reactions it is dramatic shorthand for being overwhelmed by a look, a scene, or an unexpected moment.
Also written or said as simmeot
2. The nuance
It is hyperbole in this use, not a report of cardiac arrest. For an actual medical emergency, clear literal language matters; this shorthand should not replace it.
3. Examples
“조용한 장면에서 문이 확 열려서 심멎할 뻔했어.”
The door flew open in the quiet scene and nearly made my heart stop.
“무대 조명 켜지는 순간 심멎이었어.”
The moment the stage lights came on was heart-stopping.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.