Think of an orchard the morning after a windy night. Fruit is lying on the grass, ready to pick up. Nobody climbed, nobody waited for it to ripen, nobody worked for it — the wind did the work, and whoever happens to be standing there gets to keep it. That free thing on the ground is the picture the word carries into all its uses.
Formed from wind + fall, recorded in Middle English as windfal: literally the thing the wind brought down.
這個字由 wind（風）加 fall（落下）組成，中古英語寫作 windfal，字面就是「被風吹下來的東西」。組成的兩個字都還看得出來，所以樹下的果子這個畫面很容易對上。
Wiktionary entry supplied in this packet (kaikki:3f6f69a190036ae4:899045061): From Middle English windfal, wyndfall, equivalent to wind + fall.
From the picture
突然到手的一筆錢
把樹下的果子換成掉到手上的錢，畫面就成立了：一樣沒出力、一樣沒預期、一樣是剛好輪到你。今天英文裡這一層用得遠比果子那層多。
noun · B2
Money, or another large gain, that comes to you suddenly and by luck, when you were not expecting it and did not work for it — for example an inheritance, a prize, or a payment nobody had planned on.
意外之財
noun · C1
A piece of fruit that has come off the tree by itself and is lying on the ground, usually because the wind shook it down.
風落果