1. What does it mean?
동공지진 combines 동공, the pupil of the eye, with 지진, an earthquake. In captions and conversation it paints a comic picture of someone whose eyes dart around because they are surprised, embarrassed, or unsure how to respond.
Also written or said as donggongjijin
2. The nuance
This is an expressive description, not a diagnosis of an eye condition. It can describe a reaction you imagine from a message, even when you cannot literally see the person's eyes.
3. Examples
“갑자기 발표 순서가 바뀌자 다들 동공지진이었어.”
Everyone looked completely rattled when the presentation order suddenly changed.
“케이크를 숨겨 둔 냉장고를 열어서 내가 동공지진.”
I panicked when they opened the fridge where I'd hidden the cake.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.