カクッと

adverb

with a sudden loss of strength; collapsing; slumping

Mimetic adverb describing a sudden, often momentary, loss of physical strength or balance, causing a person or thing to slump, buckle, or collapse. Often used with verbs like なる or する.

• 疲つかれて膝ひざがカクッとなった。

  My knees buckled from exhaustion.

• 彼かれは立たち上あがろうとして、カクッとバランスを崩くずした。

  He tried to stand up and suddenly lost his balance.