boundary
陲 centers on a boundary or edge, especially a border or the verge of something. The single given meaning 'boundary' captures this idea directly.
陲 combines the mound radical ⻖ (阝, often related to terrain or elevation) with 垂, which likely contributes the sound and may suggest something hanging down or a border area. The exact historical development is uncertain.
The left side ⻖ suggests a hill or raised earth, and the right side 垂 looks like something hanging down. Picture a boundary marker hanging from a hill, marking the edge of a territory.
For スイ, imagine a boundary line that swings back and forth like a pendulum: swing -> スイ, and the line marks the border.