Like a cream pie, except the woman is not on any birth control, so the cream pie turns into a birthday cake, ie: she gets pregnant.
Man: Oh man, i came in my girlfriend last night, and shes not on the pill!
Friend: Oh Dude! You gave her a birthday cake!
by Chris And Libby February 18, 2009

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When a man shits on a plate and smushes it up to look like a cake then jizz's on the top to make it look like icing. Then scrapes the 'cake' on his pubic area and makes a hole in the middle and sticks his cock in the hole. Then he makes someone eat the 'cake' and then suck his cock as a treat afterwards.
A: " John gave me a birthday cake last night, it was my favorite flavor."
b: " Really? Did you get his treat?"
A: "Yeah, it was awesome. Have you ever gotten birthday cake'd?"
b: "No, but it sounds yummy."
by heehee! March 07, 2011

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Defecating on your girlfriends chest, then ejaculating over the shit, and proceeding to lick the semen off.
A Birthday Cake is like a Cleveland Steamer, but worse.
by Zomboy October 07, 2009

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a toilet roll tube with as many joints stuck into it as you can afford/roll/fit into it. one hand placed over the end acts as a "clutch"
hey man, that was a sweet ass birthday cake we had last night
by tumbleweed_t.a. September 24, 2005

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The act of inserting a carrot rectally in a sideways fashion
My ass was sore from the birthday cake my girlfriend gave me.
by Durty Sanchez August 17, 2003

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adj.
a guy being mean to a girl
an asshole
justin!
you're being such a birthday cake
by chum August 18, 2007

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