to rise up, rainbow, to fall
隮 centers on rising up, like a rainbow arching upward, but also includes the opposite sense of falling. The meanings are contradictory, so treat it as a rare kanji with an unstable core idea.
隮 combines the mound radical ⻖ with 齊, which likely provides the sound. The exact historical development is uncertain, and the contradictory meanings suggest a complex origin.
The left mound ⻖ suggests a hill or rise, while the right side 齊 looks like a balanced, even arrangement. Picture a rainbow rising evenly from the mound, but then falling back down.
For セイ, imagine a rainbow saying 'say' as it rises: say -> セイ, and the rainbow speaks its name while arching up.