This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
岟 combines 山 (mountain) with 央, which may have contributed sound. The exact historical development is uncertain.
The mountain radical 山 suggests a connection to mountains, but the overall meaning is unclear. Picture a mountain with a central peak (央) that is hard to describe.
For ヨウ, imagine a yo-yo dangling from a mountain peak: yo -> ヨウ, tied to the mountain shape.