holy, sacred, sage, simplification of white earth, daub with white mud
垩 centers on sacredness and purity, often associated with white earth or whitewash used in ritual contexts. The meanings of holy, sacred, and sage connect to this idea of ritual purity, while daubing with white mud is a practical application.
垩 is a simplified form combining 亚 (a simplification of 亞) and 土 (earth). The original form likely related to white clay or chalk used for purification, but the exact historical development is uncertain.
The top 亚 looks like a sacred altar or vessel, and the bottom 土 is earth. Picture a holy site where white earth is used to purify and anoint, making it sacred.
For セイ, imagine a sage (セイ) daubing white mud on a sacred altar: the sage's wisdom makes the earth holy.