Hold one picture: two floors, one sitting directly on top of the other, inside a single thing. A deck is a floor you can stand on, the way a ship has decks. Put a second deck above the first and you have a double-decker — and once you can see that shape, every other use of the word is the same shape in a different material.
可以想成一個很單純的形狀：兩層樓板，一層直接疊在另一層上面，而且合起來還是同一個東西。deck 本來指船上的甲板，也就是一層站得住人的樓板。在第一層上面再加一層，就是 double-decker。抓住這個形狀之後，其他用法都只是換了材料：換成吐司、換成床、換成蛋糕，形狀還是那個形狀。
From the word’s history
Wiktionary gives the form as double + deck + -er, that is, the thing that has two decks.
來源資料寫的是 double + deck + -er，字面就是「有兩層甲板的那個東西」。-er 在這裡不是指人，而是指「帶有這個特徵的物品」。
Wiktionary etymology_text in packet: From double + deck + -er (kaikki:3f6f69a190036ae4:1401598822)
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