This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 巹 is uncertain. It visually combines 丞 and 己, but the original semantic and phonetic roles of these components are unclear.
No clear meaning is available, so a mnemonic based on shape alone is unreliable. If needed, note the top 丞 and bottom 己 as a visual pattern.
For キン, imagine a king (キン) holding a rare cup (さかずき) shaped like 巹: king -> キン.