Synonyms of 'waif'
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A foundling is a baby that has been abandoned by its parents, often in a public place, and that has then been found by someone.
Stray is also a noun.
The dog was a stray which had been adopted.
Thesaurus for waif from the Collins English Thesaurus
If you refer to a child or young woman as a waif , you mean that they are very thin and look as if they have nowhere to live .