김이 새요 · 김이 샜어요 · 김새는 소리
to lose steam; to be a letdown; to take the fun out of something
Example
영화 결말을 미리 들어서 김이 샜어요.
I heard the movie's ending ahead of time, so it was a letdown.
Word parts
Literally 'the steam leaks out,' like a pot losing its heat.
Good to know
Here 김 means 'steam,' not seaweed or the family name — a homonym trap.