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1.
Statistics prove that you have one birthday, just one birthday every year. But there are three hundred and sixty-four unbirthdays. And this is a reason to gather and cheer.
by enken January 10, 2003
 
2.
all the days besides your birthday in a year, referred to in Alice in Wonderland also
HAPPY UNBIRTHDAY TO YOU!!!
by saxaphoner September 20, 2009
 
3.
the day of a sibling's birthday where you also receive presents/money in order to keep the peace.
Jamie: Did your brother have a nice birthday?
Alexandra: He enjoyed it, I also enjoyed my unbirthday.
Jamie: How much did you get?
Alexandra: £230 and an iPad.
Jamie: Tidy.
by shr3k April 17, 2011
 
4.
When you acknowledge that it is, indeed, the date of your birth, but refuse to acknowledge that you have aged.
Even though it happens to be my eighteenth birthday today, I've decided not to become an adult.
by westindiangirl December 17, 2009