Koi is a kind of Japanese carp, but, etymologically speaking, that's like saying carp is a kind of carp. This is because koi comes from the Japanese word koi, meaning "carp" in general. This was borrowed in 1727, but only as a term for the most common type of Japanese carp- the rest all have different names. Koi in Japanese probably comes from an Old Japanese word with the same meaning and sounding like kopi (there is a citation in the Nihon Shoki, the second oldest book in Japanese). Before that, it's anyone's guess, but it probably derives from a Proto-Japonic root meaning "fish" in general or something of the sort. East Asian languages are always difficult to trace. Fun side note: in Japan, koi can also mean "love" or "affection", so koi have become synonymous with friendships and infatuations in the country. Second side note: because of the increased popularity of koi ponds in landscaping, the word koi is now at its highest usage ever.
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.