na-adjective, noun
Describes an effect or result that appears right away, often used with words like 効果 (effect) or 効き目 (efficacy).
この薬は飲むとてきめんに効く。
This medicine works instantly when you take it.
彼のアドバイスはてきめんの効果があった。
His advice had an immediate effect.
The word てきめん is derived from the kanji compound 覿面, where 覿 means 'to see' or 'to meet' and 面 means 'face' or 'surface', together suggesting something that appears immediately upon encounter. The exact historical derivation is uncertain, but the word has been used since at least the Edo period.