When it's your birthday and a man congratulates you with his penis.
"Hi John, did you have a nice birthday this weekend?"

"Yeah, Tommy came and gave me birthday dick."
by Dicktionary Uber Legend January 12, 2022
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Happens the morning after a man ejaculates and doesn't wash his penis, the result is a penis covered in frosting, just like a birthday cake.
"Ewww Johnny, I'm not sucking your penis, you have that birthday dick!"
by Rustyrastafari August 3, 2020
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Birthday dick is a really large or satisfying penis. Usually only experienced every once in a while.
After a series of sexual letdowns, Emily finally got some much needed Birthday Dick.
by Ebonykeys April 26, 2017
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