to sit
呮 centers on the act of sitting, specifically the physical action of taking a seat or resting on a surface.
呮 is a rare character. It combines 口 (mouth) with 只, but the historical development and original semantic role of these components in this character are uncertain.
The left 口 can be seen as a seat or cushion, and the right 只 suggests a person settling down. Picture a mouth-shaped cushion that you sit on.
For キ, imagine a key on a chair: you sit on the key to keep it safe. Key -> キ, and sitting on it connects to the meaning.