Picture something that is nearly complete but has holes left in the middle of it: a timber frame with empty squares, a path with gaps between the stones, a nail with a bare strip at the base, a street with an empty lot in it. Infill is whatever goes into those holes, and the work of putting it there, until the surface reads as one whole thing again.
可以想成一個幾乎完成、但中間還留著洞的東西：木框上一格一格的空白、步道石頭之間的縫、指甲根部露出來的那一條、街上一塊空著的地。infill 就是放進那些洞裡的東西，也是把它放進去的那個動作，直到整片看起來又是完整的一塊。所以不管在工地、在美甲店，還是在都市計畫圖上，這個字問的都是同一件事：中間那個缺口，要拿什麼補起來。
From the word’s history
The word is just in + fill, so its two halves already say fill into it.
這個字就是 in（進去裡面）加 fill（填滿），字面上直接說了「往裡面填」。這條線索對上它今天所有的用法：填的是磚、是石頭、是房子，動作都一樣。
Wiktionary, "infill" (etymology 1)
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