The morning on which a male wakes up and finds his cock to be much larger than the day before. Usually accompanied by fervent celebration, and parties (usually sex parties).
Todd: Dude my dick grew 1/2 an inch in girth last night!
Manuel: Congratulations man! Happy Girthday! Are you getting laid tonight?
Todd: You know it man!
Manuel: Congratulations man! Happy Girthday! Are you getting laid tonight?
Todd: You know it man!
by FuctButSects October 24, 2010
Get the Happy Girthday mug.