Picture the ground you are standing on, with the sky above it. Earthly points down, at everything that belongs to this side of the line: this life, this body, what you can touch. The same ground does a second job in negatives - not as a place you belong to, but as a place you searched. No earthly reason means you looked across the whole of it and came back with nothing.
可以想成你腳下的這片地，跟上面的天。earthly 指的是線下面這一半：這輩子、這個身體、摸得到的東西。同一片地在否定句裡換了個工作。它不再是你所屬的地方，而是你找過的範圍。no earthly reason 的意思就是整片地都翻過了，一個理由也沒帶回來。
From the word’s history
From Middle English erthely, from Old English eorþlīċ, that is earth + -ly, 'belonging to this land'.
earthly 由 earth 加上 -ly 組成，中古英文寫作 erthely，古英文是 eorþlīċ，字面是「屬於這片土地的」。今天的兩支用法都還踩在這個字面上。
Wiktionary entry for 'earthly' (packet field etymology_text): From Middle English erthely, erthlich, ierðlich, from Old English eorþlīċ, corresponding to earth + -ly.
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