cry
欷 centers on crying or sobbing, expressing grief or sorrow audibly.
欷 combines 希 (rare, hope) with 欠 (open mouth, lack), suggesting a rare or intense expression of grief through an open mouth. The exact historical development is uncertain, but the components likely contribute both sound and meaning related to sobbing.
The left side 希 can suggest a rare, precious hope, and the right side 欠 shows a person with an open mouth. Imagine someone crying out because a rare hope has been lost, their mouth wide open in grief.
For キ, picture a key unlocking a flood of tears: key -> キ, and the sound of crying fills the room.