solid
碻 centers on the idea of being physically solid, firm, or unyielding, like a rock or stone. The single given meaning "solid" captures this core sense of hardness and reliability.
碻 combines 石 (stone) with 高, which likely contributes the sound. The stone component suggests a solid, rock-like quality.
The left side 石 is a stone, and the right side 高 looks tall and imposing. Picture a tall, solid stone pillar that cannot be moved: 碻 means solid.
For カク, imagine a solid rock that makes a sharp "kak" sound when struck: kak -> カク, and the stone 石 stays firm and unbroken.