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to freeze (from fear, etc.); to be unable to move (e.g. from surprise); to be paralyzed (with horror, etc.)
to cower; to shrink in on oneself
恐怖で体がすくんで動けない。
My body is frozen with fear and I can't move.
舞台に立った瞬間に足がすくんだ。
The moment I stood on stage, my legs froze.
大きな犬を見て子供はすくんでいる。
The child is frozen at the sight of the big dog.