One bullet that finds the target by itself. It ignores everything else in its way, hits the one thing that is wrong, and the whole trouble is over in a second. That promise — perfect aim, no side damage, no waiting — is what the phrase carries wherever it goes. It is also why people so often use it to say that no such thing exists.
From the word’s history
Wiktionary records the phrase as a calque of German Zauberkugel, coined by the physician Paul Ehrlich as an allusion to the opera Der Freischütz, in which the magic bullets never miss. That never-missing bullet is exactly the picture the English phrase still carries.
來源記載這個說法是德文 Zauberkugel 的直譯，由醫師 Paul Ehrlich 造出來，典故來自歌劇《魔彈射手》(Der Freischütz) 裡百發百中的魔彈。那顆不會失手的子彈，就是今天英文裡還在用的畫面。
Wiktionary, "magic bullet"
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