Rarely written in standard Chinese, where 凼 or 塘 is used instead. It is best known in the Macau island name 氹仔 (Taipa). It is an ideograph: 水 (water) held in an enclosing stroke — water collected in a hollow.
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Dialectal and toponymic; you meet it in southern place names, not in general writing.
The single curved stroke wraps underneath like a basin. It is not really the word 乙 here but an abstract enclosure showing a hollow that holds the water above it.
