scare, frighten, intimidate
吓 centers on causing fear through aggressive action or presence: scaring, frightening, or intimidating someone.
吓 combines 口 (mouth) with 下 (down/below), likely suggesting a threatening shout or noise that presses down on someone. The exact historical development is uncertain.
A mouth 口 shouting down 下 at someone, like a loud voice coming from above to scare or intimidate them.
For カ, imagine a car suddenly honking loudly to scare someone: car -> カ, and the sound from the car's mouth frightens the person.