Made of or resembling alder wood, which comes from a type of tree commonly found near water.
From alder (Old English alor, from Proto-Germanic *aliz-) plus the suffix -n, used to form adjectives indicating composition or material. The root likely relates to the tree's wetland habitat.
Alder trees were so useful to ancient peoples that they have special wood-related adjectives in English—the wood was prized for water wheels and pilings because it resists rot when constantly wet, which is why you'd find alder groves specifically in marshy areas.
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