gakun
The moment when your energy or strength drops away all at once — knees buckling, spirits crashing, or motivation vanishing in an instant.
💡Getting bad news after working hard — 'My motivation dropped gakun when I heard the project was cancelled.'
不合格と聞いて、膝ががくんとなった。
My knees went gakun when I heard I failed.
When the final whistle blew, his legs gave way ___ and he collapsed.
試合終了のホイッスルが鳴った瞬間、足が○○となった。