면 망하다

if you do that, it's ruined

망하다 is going under, flopping, being ruined, and -(으)면 supplies the condition that would cause it. It runs from literal business failure (전기차가 많아지면 망하는 거고, this gas station goes bust if electric cars take over) to any plan going wrong (이거 이름을 그걸로 하면 망한다, name it that and it's dead). The word is blunt — fine among friends, not something to say about someone's company to their face.

Register: casual and blunt