一拳必殺 names a karate ideal: one technique that decides the fight. It asks for everything a person has in a single blow, and the spirit that trains for it. The karate teachers who use it are plain that killing is not the aim. The aim is to end a fight in the shortest time. Character by character it is one fist, then certain kill; in a dojo they mean the decisive technique. A person who chooses them chooses that creed.
一撃必殺, ichigeki hissatsu, the same ideal with the strike named, is sold here as its own design.
The English follows Nakayama Masatoshi's, from his Best Karate of 1977. 一拳必殺 is composed of the kanji 一 (read ichi) meaning "one", 拳 (read ken) meaning "fist", and the word 必殺 (read hissatsu) meaning "certain kill".
一拳必殺 is a karate phrase, in print from 1934 and by 1936 a saying in karate, where it stands beside its twin 一撃必殺, one strike; karate kept it after the war, and it reached English as ikken hissatsu in Nakayama's Best Karate of 1977.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 一拳必殺.