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1.
The (lame) double gift. Usually for a person unfortunate enough to be born on or around Christmas.
I was born on 12/23 and never get separate birthday and x-mas gifts. I always get some lame birthmas gift.
by BuddhaBrother June 27, 2005
 
2.
The period from about December 20th - January 5th in which a person's birthday may fall and they get a "Christmas/Birthday Present". I.E. They get one present for their birthday and Christmas instead of two due to their birthday falling so close to the holiday.
I got screwed again on presents this year...my mom decided to get me a birthmas present to make her holiday shopping easier and cheaper.
by Michelle Davies December 28, 2008
 
3.
The birthday of a person that happens to coincide with December twenty-fifth, or Christmas day.
Jeremy is one of the few people I know who has a birthmas.
by Alcabum November 15, 2009
 
4.
-The date of a persons birth.
-Another word for birthday.
Merry birthmas to me!
by daymare June 11, 2005