This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
杦 is a rare character whose historical origin is uncertain. It visually combines 木 (tree) and 久, but the exact development is unclear.
The meaning is unclear, but the shape shows a tree 木 beside 久, which might suggest something long-lasting like an old tree.
For すぎ, imagine a tall cedar tree (杉, すぎ) that looks similar: 杉 -> すぎ, and 杦 also has a tree 木 on the left.
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