Picture a knife going into a and coming out with a fat slice, or butter going onto that slice in a deep, generous layer. Thickly is the word for that amount: more of it, piled up, not spread out thin. Now stand back until the is a whole , and the same word describes trees standing shoulder to shoulder across it.
Wiktionary gives Middle English thikly, thick plus the adverb ending -ly, which turns an adjective into the manner of doing something.
Wiktionary 記載這個字在中古英語就寫作 thikly，是 thick 加上副詞字尾 -ly。-ly 的作用是把形容詞變成「用那個方式」，像 quick→quickly。所以它一直就是「以很厚的方式」。
Wiktionary entry for thickly, packet source_ref kaikki:3f6f69a190036ae4:1026512684
From the picture
一片地上擠得很密
厚的意思是「同一種東西疊在一起、數量很多」。把這個疊放大到一整片土地上，疊的就不是奶油，而是樹和房子和人——所以英文照樣用 thickly。這是概念上的連結，不是說一個意思從另一個衍生出來。
r · B2
Used before words like populated, wooded, settled or forested to say that an area has a lot of people, trees or houses in it, standing close together.
（人口、樹林）密集地
r · B2
In a way that makes a thick piece or a thick layer of something: cut into fat slices, or spread, covered or laid on in a deep layer.
厚厚地、厚厚一層地