This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 窼 is uncertain. It visually combines 穴 (hole, cave) with 巢 (nest), but the original semantic and phonetic relationships are unclear.
The top 穴 suggests a hole or cave, and the bottom 巢 looks like a nest. Picture a hidden nest inside a cave, though the kanji's actual meaning is uncertain.
For ソウ, imagine a soul (ソウ) of a bird nesting inside a cave: soul -> ソウ, and the nest 巢 sits under the cave 穴.