Girl 2: Nah, I just haven't pooped for 3 weeks.
Girl 1: Oh I see. You have a shit-baby.
Girl 2: Well yes I do.
The rare and unfortunate occurrence of a bowel movement that crests the leg openings AND rear band of an infant or toddler's diaper, resulting in yellow, curdy staining of a substantial area of the child and his/her clothing (and often furniture, parents, and household pets).
See also shitback and shitleg
Chad: That's gross.
Jen: Seriously, it's going to be the size of a child. I have to go.