This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 靃 is uncertain. It visually combines 雨 (rain) with 雔 (a pair of birds), but the original semantic or phonetic motivation is unclear.
With 雨 on top and 雔 (two birds) below, imagine rain falling on a pair of birds huddled together, but the meaning remains obscure.
For カク, picture a pair of birds under the rain making a harsh 'kak' sound as they shake off water: kak -> カク.