This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 晵 is uncertain. It visually combines 日 (sun) with an upper element that resembles 戸 and 攵, but the original composition and meaning are unclear.
With no clear meaning, focus on the shape: 日 (sun) sits under a structure that looks like 戸 (door) and 攵 (striking hand). Imagine sunlight breaking through a door that has been struck open.
For ケイ, picture a key (ケイ) unlocking the door-like upper part to let the sun 日 shine through: key -> ケイ.