sluice, spout, floodgate, penstock, (kokuji)
杁 centers on a water-control structure: a sluice, floodgate, or penstock that directs flow. The spout sense is a natural extension of that controlled release.
杁 is a kokuji (Japan-made kanji) combining 木 (wood) and 入 (enter), likely suggesting a wooden structure that lets water in or through.
Wood 木 on the left and enter 入 on the right: picture a wooden gate that lets water enter a channel—a sluice or floodgate.
For いり, think of water entering (入り, いり) through the wooden gate: 入り -> いり, and the gate controls the flow.
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