|a person or thing, initially ugly or unpromising, that changes into something beautiful or admirable|
|[an allusion to The Ugly Duckling, a story by Hans Christian Andersen]|
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.