a foster child never truly becomes one's own
Proverb meaning that no matter how lovingly you raise a foster child, they are not your biological child. Rare and literary.
There is a proverb: 'A foster child never truly becomes one's own.'
Alternate kanji spelling using 時鳥 for ほととぎす; less common.
The exact origin is uncertain; it is a traditional Japanese proverb using the image of the lesser cuckoo (ほととぎす) as a metaphor for a child that does not become one's own.