hole, cleft, empty
竉 centers on an opening or void: a hole, a cleft, or an empty space. The meanings are closely related, all describing a physical gap or absence.
竉 combines 穴 (hole, cave) with 龍 (dragon), likely as a phono-semantic compound where 龍 contributes the sound and 穴 gives the meaning of an opening or cavity.
A dragon (龍) lives inside a cave (穴), which is a large hole or cleft in the mountain. The cave is empty except for the dragon.
For ロウ, imagine a dragon in a low cave: low -> ロウ, and the dragon's home is a hole deep and low in the earth.