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1.
Similar to a birthday, except instead of celebrating the day a man was born, he celebrates the day in which his penis reached its maximum girth.
Hey ladies!! Today is my girthday, and i was hoping you'd celebrate with me by letting me go deep inside! Whaddaya say?
by MikeyB1980 January 16, 2014
 
2.
Girth Day occurs every May 1. It is a day to honor those among us that are Girthier than the average man. Examples of honorees include: Arnold Schwarzenegger, Jean Baptista, and Ron Jeremy.
Happy Girth Day!
by jmn3813 April 30, 2011
 
3.
The day by which you loose ur virginity.

derived from the words girth and birthday...girth being the thinkness of the penis
jordan: happy girthday!
andrew: cheers mate, i cant believe its been 9 years now!!
happy birthday penis
by <<*ROoOo*>> August 14, 2008