1. What does it mean?
갑툭튀 shortens 갑자기 툭 튀어나오다. It can describe an unexpected person, a pop-up, a startling scene, or a plot element introduced without much preparation.
Also written or said as gaptuktwi
2. The nuance
It is not limited to horror jump scares. The reaction can be amused or annoyed; in a story review, 갑툭튀 may criticize the lack of setup rather than the new character themselves.
3. Examples
“산책 중에 친구가 골목에서 갑툭튀해서 놀랐어.”
My friend popped out of a side street during my walk and startled me.
“마지막 회에 새 설정이 갑툭튀해서 이해가 안 됐어.”
A new plot device appeared out of nowhere in the final episode, and I couldn't follow it.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.