This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 坹 is uncertain. It visually combines 土 (earth) and 穴 (hole), but whether this reflects the original meaning or is a later simplification is unclear.
The kanji shows earth 土 beside a hole 穴. Picture a hole in the ground, but since the meaning is uncertain, treat this as a visual reminder only.
For ケツ, imagine a deep hole in the earth that looks like a giant kettle: kettle -> ケツ, and the hole is in the ground.
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