Picture a place to sit whose whole point is the window beside it. The chair itself does not matter much; what matters is that when you sit down, the outside is right there. On a plane or a bus the view slides past you. In a room the view stays still and you stay with it. Same two words, and the room you are in tells you which one someone means.
可以想成「一個位子，重點在旁邊那扇窗」。椅子長什麼樣不重要，重要的是坐下去就看得到外面。在飛機或公車上，窗外的景色一直往後跑。在房子裡，景色不動，是你待著不動。兩個意思用的是同樣兩個字，靠場景就分得出來。
From the word’s history
Wiktionary records the compound simply as window + seat.
維基詞典把這個詞記成 window + seat，就是字面上的「窗＋座位」，兩個用法都貼著這個字面走。
Wiktionary, "window seat"
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