Instantly; in the spur of the moment; a reflexive or immediate reaction when there is little time to think.
ボールが飛んできたのでとっさに手で受け止めた。
A ball flew at me so I caught it with my hand instinctively.
危ないと思ってとっさにブレーキを踏んだ。
I thought it was dangerous and hit the brakes immediately.
とっさに出た言葉だったが、相手を傷つけてしまった。
It was something I said on the spur of the moment, but it ended up hurting the other person.