decayed tooth, cavity
齲 centers on tooth decay: a decayed tooth or a cavity, the result of dental rot.
齲 combines 齒 (tooth) with 禹, which likely contributes the sound. The character expresses tooth decay or cavities.
The left side is a tooth 齒, and the right side 禹 looks like a worm or bug inside a container. Imagine a worm burrowing into a tooth, creating a decayed cavity.
For ク, picture a cool, refreshing drink that soothes the pain of a decayed tooth: cool -> ク, but the cavity still needs a dentist.