I’m going to add a bit more semi-related info. Keep in mind what I am about to say is absolutely not necessary for learning/understanding Japanese, so consider this “just for fun”.
What we now call い-adjectives used to be more complicated. At one point that sort of adjective could be divided into シク活用 (“shiku” inflection) and ク活用 (“ku” inflection). In modern Japanese you can sort of see this when inflecting something like 美しい vs 高い, they become 美しく vs 高く.
In modern Japanese い-adjectives look identical in their 終止形 (there are many translations for this but: terminal form/plain form/dictionary form/predicative form), which is the form at the end of sentences, and in their 連体形 (attributive form), which is the form before a noun. So we have 山が高い and 高い山, both with 高い looking exactly the same.
In classical Japanese the above two forms were different. The 終止形 (terminal form) looked like 高し, and the 連体形 (attributive form) looked like 高き. This means you’d have phrases like 山高し and 高き山. But 高し (高い) was a ク活用 adjective. With a シク活用 adjective you would have a pattern like 花美し and 美しき花. The fact that the し was not dropped but remains part of the inflectable part of the word is what makes it distinct. By the way, you can still find なし (ない) hanging around in modern Japanese, with its classical 終止形 (terminal form).
Over time many different things happened, including sound changes. To keep it brief, the し and き sounds in these forms changed to い and thus the forms now look identical in modern Japanese, and the difference between シク活用 and ク活用 dissapeared as adjectives became unified by this change. い became the important factor, not the behaviour of し.
The above is roughly what a high schooler would have to learn about this topic, although actual details about origin become a messy argument as it stretches beyond the historical record. You can have a look at what a high schooler might study about this here. Let’s just say that it is probably easier to learn modern Japanese grammar.
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