adjoin
堧 centers on the idea of land or earth that is adjacent or attached to something else, such as adjoining land or a border area.
堧 combines 土 (earth) with 耎, which likely contributes the sound and may relate to softness or yielding ground, but the exact historical development is uncertain.
The left side 土 is earth, and the right side 耎 looks like a large 大 person with a soft 而 head. Picture soft earth that adjoins a harder surface, like a border of yielding soil.
For ゼン, imagine a Zen garden where carefully raked gravel adjoins a smooth stone path: Zen -> ゼン, and the earth borders the stone.