privy hole
牏 refers to a privy hole, a specific architectural feature for waste disposal. Its meaning is narrow and concrete, with no broader conceptual center.
牏 combines 片 (split wood, board) with 俞, which likely contributes the sound. The exact historical development is uncertain, but the character probably originally referred to a wooden board with a hole used as a privy.
The left side 片 suggests a wooden board, and the right side 俞 looks like a hole with a cover. Picture a wooden board with a hole cut into it, used as a privy.
For チュ, imagine a privy hole that makes a 'chew' sound when something falls through: chew -> チュ, and the hole is in the wooden board.