an unattractive or unpromising child who becomes a beautiful or much-admired adult.
Origin
1880-85; after the bird in the story of the same name by Hans Christian Andersen
Examples from the web for ugly duckling
But invasive species aren't always an ugly duckling and emotions can run high against efforts to remove them.
He was the ugly duckling where he came from, and he's worshipped here.
The ugly duckling becomes a swan and exacts her revenge in this complex drama.
It must have been extremely difficult to be the ugly duckling in a family of swans.
Indeed, it's the ugly duckling who has turned into a swan.
Inside those walls she would forever remain the ugly duckling of her early teens.
British Dictionary definitions for ugly duckling
ugly duckling
noun
1.
a person or thing, initially ugly or unpromising, that changes into something beautiful or admirable
Word Origin
an allusion to The Ugly Duckling, a story by Hans Christian Andersen
Idioms and Phrases with ugly duckling
ugly duckling
A homely or unpromising individual who grows into an attractive or talented person, as in She was the family ugly duckling but blossomed in her twenties. This term alludes to Hans Christian Andersen's fairy tale about a cygnet hatched with ducklings that is despised for its clumsiness until it grows up into a beautiful swan. The tale was first translated into English in 1846, and the term was used figuratively by 1871.