This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The character combines 鹿 (deer) with 京, but the original semantic or phonetic role of 京 is unclear.
A deer 鹿 carrying a capital 京 on its back? The meaning is unclear, so this mnemonic is just a visual placeholder.
For ケイ, imagine a deer 鹿 wearing a cape ケイプ (cape) while standing in the capital 京. The cape cues ケイ.