I have to admit that the word 땅거미 confused me for the longest time. I could never wrap my head around what exactly "거미" (spider) has to do with "twilight." It turns out that there is a very simple explanation for the etymology of this word which will make it much easier to remember if, like me, you have been having a hard time connecting "spiders" and "twilight."
땅거미 can be broken into two parts: 땅 + 거미. 땅 means "earth, land," and while 거미 is spelt in exactly the same way as the word for "spider," it is in fact analyzed as follows: 검(다) + 이 (noun suffix), where 검다 means "black, dark." The word 땅거미 refers to the time of day when the sun goes down and the land grows dark, hence the meaning "twilight."
백문식. 우리말 어원 사전. 1st ed. 서울: 도서출판 박이정, 2014. Print.