instrument
柷 refers to a specific ancient percussion instrument, a wooden box-shaped idiophone struck with a mallet to mark the beginning of music in ritual contexts.
柷 combines 木 (wood) with 兄, which likely contributes the sound. The character denotes a wooden percussion instrument, and the phonetic component 兄 suggests the reading.
The left side 木 indicates wood, and the right side 兄 looks like a person striking something. Picture a wooden box being struck by an elder brother (兄) to start a ceremony.
For シュク, imagine the instrument being struck with a shoe-like mallet: shoe -> シュ, and the sound echoes 'kuk' as the box resonates. Together, shoe-kuk -> シュク.
—