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at once; immediately; instantly; in an instant; quickly; suddenly; on the spur of the moment
とっさに体が動いて子供を助けた。
My body moved instantly and I saved the child.
彼はとっさに嘘をついてしまったらしい。
It seems he instantly told a lie.
危険を察知してとっさに身をかわすことができた。
I sensed the danger and managed to dodge on the spot.