This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 盁 is uncertain. It visually combines 乃 and 皿, but the original formation is unclear.
With no clear meaning, picture an empty dish 皿 with a curved object 乃 above it, like a mysterious container waiting to be filled.
For エイ, imagine an 'A'-shaped object (エイ sounds like 'A') resting on a plate 皿: the 'A' on the dish cues エイ.