This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 牃 is uncertain. It visually combines 爿 and 枼, but the origin and original meaning are unclear.
Since the meaning is unclear, focus on the shape: a bed or board 爿 beside a tree-like 枼. No reliable meaning mnemonic can be given.
For チョウ, imagine a long-serving chief named Chou leaning a board 爿 against a tree 枼: Chou -> チョウ.