You can picture thinly as a knife going down through a : whatever comes off has almost nothing between its two sides. Hold that one picture and the rest of the word follows. A layer of butter with almost nothing to it. Houses spread over so much land that there is almost nothing in any one spot. A cover over someone's real feeling with almost nothing to it, so you look straight through. Each time, thinly is answering the same question: how little is there between here and the other side?
From the word’s history
Wiktionary records thinly as thin + -ly, going back to Middle English þynnelich. The adjective describes the thing; the -ly ending turns it into a description of how an action came out.
字典記錄 thinly 就是 thin（薄）加上副詞字尾 -ly，往前可以追到中古英語的 þynnelich。形容詞講東西本身薄，加了 -ly 之後，改成講「這個動作做出來的結果很薄」。
Wiktionary entry for thinly (supplied in evidence packet): "From Middle English þynnelich; equivalent to thin + -ly."
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