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at once; immediately; instantly; in an instant; quickly; suddenly; on the spur of the moment
とっさに危険を避けることができた。
I was able to avoid danger in an instant.
とっさに何と言えばいいか分からなかった。
I didn't know what to say in the moment.
とっさに身をかわして飛んできたボールを避けることに成功した。
I instinctively dodged and successfully avoided the ball that came flying.