narrow, obstruct
隘 centers on a passage that is too tight or blocked: a narrow defile, a bottleneck, or an obstruction that hinders movement.
隘 combines ⻖ (hill, mound) with 益, which likely contributes the sound and the idea of a narrow or obstructed passage between hills.
The left side ⻖ suggests a hill or earthen wall, and the right side 益 looks like a container with a lid. Picture a narrow mountain pass where the hills squeeze the path like a tight lid on a box.
For アイ, imagine an eye (アイ) peering through a narrow crack in a wall: the eye is obstructed by the tight space, just as 隘 means narrow and obstructed.