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1.
When someone was ugly when they were young and is now sexy/looking good. They have ugly duck syndrome as they believe to be still ugly but are actually very good looking usually works well for the partner who met them when they were ugly. Ugly Duck Syndrome is a good looking person who lacks confidence or releases they are much hotter than they think and there partner but the partner is aware she has ugly duck syndrome.

Much like the tale of the ugly duck who turned in to a swan.
Wow she is so hot and does not realise it, she has ugly duck syndrome.

Man 1: I met a girl who is hot and she actually like me.
Man 2: She must have Ugly Duck Syndrome (UDS)
by Mr Thinker. November 13, 2013