Nowadays he have human milk banks. In the olden days, however, it was not unusual to see a woman nurse the child of another mother who couldn't produce her own milk. Is there a word or phrase for a woman who did it and was not biologically related to the child?
They're called wet nurses. Wikipedia
A wet nurse is a woman who breast feeds and cares for another's child. Wet nurses are employed when the mother is unable or chooses not to nurse the child herself.
Wet-nursed children may be known as "milk-siblings", and in some cultures the families are linked by a special relationship of milk kinship. Mothers who nurse each other's babies are engaging in a reciprocal act known as cross-nursing or co-nursing.
- For years it was a really good job for a woman. In 17th- and 18th-century Britain a woman would earn more money as a wet nurse than her husband could as a laborer. And if you were a royal wet nurse you would be honored for life.