Definition of ugly duckling noun from the Oxford Advanced American Dictionary

 

ugly duckling

 noun
noun
 
 
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a person or thing that at first does not seem attractive or likely to succeed but that later becomes successful or much admired From the title of a story by Hans Christian Andersen, in which a young swan thinks it is an ugly young duck until it grows up into a beautiful adult swan.
See the Oxford Advanced Learner's Dictionary entry: ugly duckling