Say wham out loud and you have already got the word: something lands hard, fast, and out of nowhere. A whammy is one of those hits, given a name. Usually the hit is bad luck, and usually it arrives with a friend, which is why the double whammy is the form everyone knows. Sometimes the hit is aimed on purpose, like a curse. And on an electric guitar, the thing that gets hit is the note itself.
把 wham 念出聲，這個字的畫面就到了：有東西又快又重地撞上來，而且毫無預告。whammy 就是把那一下取了名字。撞上來的多半是衰事，而且很少單獨來，所以大家最熟的形式是 a double whammy。有時候那一下是有人故意朝你丟的，像一道咒。至於電吉他上，被撞歪的不是人，是那個音本身。
From the word’s history
Whammy is wham plus the ending -y, so the sound of a sudden hard hit is built into the word.
whammy 由狀聲詞 wham（猛力撞一下的聲音）加上字尾 -y 組成，所以「突然重重挨一下」這個感覺是從聲音直接長出來的，不是後來才附加的比喻。
Wiktionary, "whammy"
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