shape wood by use of heat, tool for shaping bent wood
櫽 centers on shaping wood using heat: a specialized tool or process for bending wood into curved forms.
櫽 combines 木 (wood) with 隱, which likely contributes sound and the idea of bending or hiding. The exact historical development is uncertain, but it is a phono-semantic compound related to woodworking.
The top 隱 suggests something concealed or bent, and the bottom 木 is wood. Picture a hidden workshop where wood is heated and bent into shape.
For イン, imagine an inn where craftsmen bend wood by the fireplace: inn -> イン, and the warm inn shapes the wood.