One way to hold this word is the cuckoo's nest. A cuckoo lays its egg in another bird's nest and flies off, and the other bird sits on it and feeds it, never knowing. English took that picture and pointed it at a husband: the man at home with something in his nest that is not his, and everyone watching except him.
這個字可以想成布穀鳥的巢。布穀鳥把自己的蛋下在別的鳥巢裡就飛走了，那隻鳥照樣孵、照樣餵，一直不知道。英文把這個畫面搬到婚姻上：丈夫還在家裡，巢裡卻多了不屬於他的東西，而旁邊的人都看得見，只有他不知道。名詞是那個丈夫，動詞是對他做出這件事。也因為畫面裡最好笑的是那隻被瞞著的鳥，這個字罵的才會是被騙的人。
From the word’s history
Wiktionary derives cuckold from Old French cucuault, built on cucu 'cuckoo', naming the bird's habit of leaving its eggs in another nest.
這份資料說 cuckold 來自古法文 cucuault，字根 cucu 就是「布穀鳥」，指的正是牠把蛋下在別人巢裡的習性。所以這個核心畫面不是後人硬掰的比喻，字本身就是從那隻鳥來的。
Wiktionary, "cuckold"
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