Wet Behind The Ears
This phrase is used to state that someone is very young or, more often, inexperienced. When animals are born, they are wet from the amniotic fluid and the birthing blood. After giving birth, the mother animal licks her offspring clean with her tongue. This also stimulates them into taking their first breath. At that point, they begin to nurse and their fur dries. Due to the folds of skin, the last place to dry is often behind the ears, at the base. Thus, they are so newly born that they are still wet behind the ears.

(来源：英语博客 http://space.englishcn.com)