hidden, mysterious, secret, to conceal, small, minute, fishhook radical variant (no. 5)
乚 centers on concealment and smallness: something hidden, secret, or minute. The fishhook radical sense is a specialized use of the same shape as a component.
乚 is a pictograph of a bent or hidden object, often interpreted as a hook or a concealed thing. Its exact historical development is uncertain, but the shape suggests something small and tucked away.
The single stroke bends and hides itself like a secret tucked into a corner. Picture a tiny fishhook concealed in the palm of your hand.
For イン, imagine a hidden inn where secrets are kept: inn -> イン, and the tiny hook-shaped sign marks the secret entrance.