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1.
The child born to a woman who had unprotected anal sex. The semen from the rectum drained (throughout daily activities, standing, walking, etc.) and eventually made its way down to her vagina, where it proceeded to impregnate her. It is possible. Mind you.
I fucked Mandie in the ass without a condom last night. She said it leaked and got in her pussy, she might have a butthole baby.
by Rob October 02, 2003
 
2.
When you are dropping a deuce, and it is so wide it feels like you just pooped out a small baby.
"Wow Jon, I just took a crap and I think I may have done some permanant damage with that last butthole baby."
by Aaron Wassman May 02, 2007