This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
窐 combines 穴 (hole, cavity) with 圭, but the exact historical development is uncertain. It may have originally related to a hollow or empty space, but this is speculative.
The top 穴 suggests a hole or empty space, and the bottom 圭 looks like stacked earth. Imagine a hollow in the ground, but the exact meaning is unclear.
For ケイ, picture a cave (穴) with a key (ケイ) shaped like 圭 inside: key -> ケイ, but the cave is empty and its purpose is a mystery.