말짱 도루묵 되다: Something you put in a lot of work falling through.

말짱: everything (usually used with negative expressions)

도루묵: A Korean name for a hard-finned sandfish.

→ When we say 말짱 도루묵 되다, it means everything you’ve worked for went down the drain like the fish’s name once changed to a silverfish and then became 묵 again.