pit, vault, store in pit
窊 centers on a hollow space: a pit, a vault, or a place for storing things underground. The idea of a sunken cavity connects all its meanings.
窊 combines 穴 (hole, cavity) with 瓜 (melon), likely suggesting a melon-shaped pit or a hollow like a melon's interior. The exact historical development is uncertain.
A hole 穴 with a melon 瓜 inside: imagine a pit dug to store melons, like a root cellar. That pit is a vault for keeping things.
For ワ, picture a melon rolling into a pit with a soft 'wah' sound: wah -> ワ, as it drops into the vault.