Having an immediate, striking effect; instantaneous or prompt in producing results (as in 'highly effective').
この薬は頭痛にてきめんに効いた。
This medicine worked instantly for my headache.
早寝をしたら、疲れがてきめんに取れた。
After going to bed early, my fatigue was promptly relieved.
彼のアドバイスは問題解決にてきめんだった。
His advice was instantly effective in solving the problem.