small mound, place name, hillock
垞 centers on a small raised piece of land: a hillock or small mound. The place-name sense is a natural extension, as such features often appear in location names.
垞 combines 土 (earth) with 宅, which likely contributes the sound タ. The character is used for a small earthen mound or hillock.
The earth radical 土 beside 宅 suggests a small earthen rise near a dwelling. Picture a tiny hillock next to a house.
For タ, imagine a small mound of tar beside a house: tar -> タ, and the sticky earth forms a hillock.
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