A vagina, obviously. Can be used in many situations especially when a teacher is around and you need to use code. Also there is a handmotion that goes along with it that is characterized by cupping both hands together and folding and unfolding them on the hinge of the outer edge of your hands. This should resemble a vagina. This handmotion can be used to replace the popular "awkward turtle" handmotion but mostly is used in context of sexual innuendo. Synonyms include: stickbag, chamber of secrets, and the ever popular "danger zone"
"That was such an awkward baby pocket moment."
"My baby pocket itches"
"Reindeer baby pocket!"