to yawn
呿 centers on the act of yawning: opening the mouth wide, often involuntarily, as a sign of tiredness or boredom.
呿 combines 口 (mouth) with 去, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a mouth-related action.
The left side is a mouth 口, and the right side 去 looks like a person stretching away. Picture a mouth opening wide as if trying to leave the face during a big yawn.
For カ, imagine a car (カ) with a sleepy driver whose mouth opens in a huge yawn: car -> カ, and the yawn escapes through the open mouth.