An ugly duckling is a plain child who grows into a beautiful woman.
This idiom has its origin in the fairy tale The Ugly Duckling by Danish author Hans Christian Anderson (1805-75).
The ugly duckling in Hans Anderson’s story is teased mercilessly by neighbouring birds because it is unattractive, but at the end of the story it grows into a beautiful swan.
Examples of use:
1. She’s a beautiful woman, but we knew her when she was a girl and a real ugly duckling.
2. Alice’s daughter is a bit of an ugly duckling now but, like her mother, she will grow to be a very attractive woman.
Image by Ouf-Ouf’s World