skull
髏 centers on the skull, the bony structure of the head.
髏 combines 骨 (bone) with 婁, which likely contributes the sound ロ. The character specifically denotes the skull.
The left side 骨 means bone, and the right side 婁 looks like a head with a top and a face-like middle. Together they form the bony head: the skull.
For ロ, imagine a low, hollow sound echoing inside a skull: low -> ロ.
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