Referred to when a female (or male) has been out on the lash and begins to chunder. She is sitting down with her legs apart in the same way a woman would sit when giving birth and is being sick between them. As if the chunder is her new born baby.
Girl: I was sat on the street puking between my legs on the road I think... very classy
Dude: Sounds like you had a Chunder Baby last night!