1. What does it mean?
깜놀 is a contraction of 깜짝 놀라다. It names the instant reaction when an unexpected sound, appearance, message, or piece of news catches someone by surprise.
Also written or said as 깜놀하다, 깜짝 놀라다
2. The nuance
It works well in short captions and casual retellings, including the verb form 깜놀했다. The surprise may be pleasant or unpleasant. For a serious incident, a fuller explanation tells the listener more than this light shorthand.
3. Examples
“조용한 방에서 알람이 울려서 깜놀했어.”
The alarm went off in the quiet room and gave me a fright.
“오랜만에 온 친구가 문 앞에 있어서 깜놀했지.”
I was so surprised to find a friend I hadn't seen in ages at the door.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.