skull
髑 centers on the skull, the bony structure of the head.
髑 combines 骨 (bone) with 蜀, which likely contributes the sound ドク. The character specifically denotes the skull.
The left side 骨 means bone, and the right side 蜀 looks like a head with a spine and a cap. Picture a bony head: a skull.
For ドク, imagine a doctor examining a skull: doc -> ドク, and the doctor taps the bony skull.
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