From Old Korean, ultimately derived from Baekje 吉支 (*kici, “ruler”) or at least cognate to the Baekje form. The vowel quality of the second syllable is probably an irregular sound shift caused by conflation with Chinese 子, Middle Korean reading ᄌᆞ (co).
The existence of the Old Korean ancestor is implied by a quotation in the Goryeosa, the official history of the Goryeo dynasty (918—1392), that the ninth-century priest Doseon made a comment alluding to kingship by referring to the millet plant. The quote in the Goryeosa states that the words for "millet" and "king" sounded similar in the Korean language of the time. "Millet" in Middle Korean is 기자ᇰ (kìcàng).
긔ᄌᆞ (kuyco)
긔ᄌᆞ is not found in any running text, suggesting that it was extremely rare or unknown in at least the Seoul dialect which represents the vast majority of the Middle Korean corpus. It only appears in various editions of the 1575 Gwangju cheonjamun, a Korean version of the Chinese poem Thousand Character Classic with a Middle Korean gloss for each Chinese character, where this word is used to gloss Chinese 王 (“king”). The Middle Korean glossing tradition is known for linguistic conservatism, and this word may have survived due to this.
Alternatively, the Gwangju cheonjamun was published in Gwangju, a southwestern city which was formerly the territory of the kingdom of Baekje. Therefore, it may reflect a dialectal form which survived in the ancient lands of Baekje.
The conventional Middle Korean gloss for 王 (“king”) is 님〯금〮 (nǐmkúm), which is also attested commonly in running text and is still found in Korean as 임금 (imgeum). Interestingly, the Gwangju cheonjamun still glosses the Chinese words 皇 (huáng, “emperor”), 帝 (dì, “emperor”), 聖／圣 (shèng, “sacred”), and 御 (“imperial”) as 님금. In East Asia, rulers of the title 王 are politically subordinate to rulers of the titles 皇 (huáng) and 帝 (dì), suggesting that 긔ᄌᆞ may have referred to a specific sort of ruler who was subordinate to a supreme sovereign. (In conventional Korean glosses, 王, 皇 (huáng), and 帝 (dì) are all glossed as 님금 without distinction.)