clear, serene, cold
刳 centers on cutting or hollowing out, which can extend metaphorically to clearing away or making something empty. The given meanings "clear," "serene," and "cold" likely derive from the idea of a hollowed-out, still, and chilly space.
刳 combines 夸 (likely a phonetic component) with 刂 (knife), suggesting cutting or hollowing out. The exact historical development is uncertain, but the knife radical points to an action involving cutting.
The left side 夸 looks like a big 大 with something inside, and the right side 刂 is a knife. Imagine using a knife to hollow out a large object, leaving it clear, serene, and cold inside.
For コ, imagine a cold, hollowed-out core: core -> コ, and the knife cuts out the core to make it clear.