old name for a Chinese mountain, Taishan
岱 is a proper noun referring to Mount Tai (Taishan), a historically significant Chinese mountain. Its meaning is tied to this specific place name rather than a general concept.
岱 combines 代 (which may contribute a phonetic element) with 山 (mountain), directly indicating a mountain name. The character was created specifically to write the name of Mount Tai.
The top part 代 can be thought of as 'generation' or 'substitute', and the bottom is 山 (mountain). Imagine a mountain so revered it has been a substitute for heaven across generations: that is 岱, Mount Tai.
For タイ, picture tying a prayer ribbon at the mountain: tie -> タイ, and you tie your wishes on the sacred Mount Tai.
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