This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 耺 is uncertain. It visually combines 耳 (ear) with 云, but the original semantic or phonetic role of 云 in this character is unclear.
The left side is an ear 耳, and the right side 云 looks like a cloud. Imagine a cloud whispering into an ear, but the message is too faint to understand—just like the meaning of this kanji.
For ウン, imagine the sound of a faint ringing in the ear: a low 'unnn' hum. The ear 耳 hears the cloud 云 making an 'unnn' sound: ウン.