mast crossbeam
棡 refers to a specific structural part of a ship: the mast crossbeam, a horizontal timber that supports the mast.
棡 combines 木 (tree/wood) with 岡 (hill/mound), likely as a phono-semantic compound where 岡 contributes the sound コウ and 木 indicates the material (wood). The specific application to a mast crossbeam is a specialized nautical usage.
The left side 木 is wood, and the right side 岡 looks like a sturdy frame. Picture a wooden crossbeam bracing the mast of a ship, as solid as a hill.
For コウ, imagine a ship's mast crossbeam made of strong 鋼 (こう, steel) instead of wood: 鋼 -> コウ, and the beam holds the mast firm.
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