とっさに
とっさに · tossani · JLPT N1 · adverb
at once
⚡ 突 (sudden) + 嗟 (instant): とっさに = in a sudden instant, with no time to think
突 means sudden or thrust, and 嗟 means a brief moment. Together they describe a reaction that happens right away, before any deliberate thought. Sounds like toss-a-knee: you react without planning.