This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 昖 is uncertain. It visually combines 日 (sun) and 公, but the original meaning and component relationship are unclear.
With no clear meaning, picture the sun 日 beside a public square 公, but the scene is empty and undefined.
For エン, imagine the sun 日 shining on an empty public square 公, and a voice echoes 'en' across the space: en -> エン.