It’s a classicism. In classical Japanese, the ending of い adjectives (called くadjectives back then) became き when the adjective appeared in front of a noun. In sentence-final form it was し, but you don’t see that much anymore.
Edit: More info [here](https://www.imabi.net/classicaladjectivesi.htm). It’s kind of like using old-timey biblical language like “shalt,” “didst,” and “thee/thou,” but this is an imperfect analogy, because that kind of thing fell out of style centuries ago and is now mostly used as a joke, while classical Japanese was the standard literary language well into the 20th century.
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It’s a classicism. In classical Japanese, the ending of い adjectives (called くadjectives back then) became き when the adjective appeared in front of a noun. In sentence-final form it was し, but you don’t see that much anymore.
Edit: More info [here](https://www.imabi.net/classicaladjectivesi.htm). It’s kind of like using old-timey biblical language like “shalt,” “didst,” and “thee/thou,” but this is an imperfect analogy, because that kind of thing fell out of style centuries ago and is now mostly used as a joke, while classical Japanese was the standard literary language well into the 20th century.