wet behind the ears

Also, not dry behind the ears. Immature, inexperienced, as in How can you take instructions from Tom? He's still wet behind the ears, or Jane's not dry behind the ears yet. This term alludes to the fact that the last place to dry in a newborn colt or calf is the indentation behind its ears. [Early 1900s]

The adjective has one meaning:

Meaning #1: lacking training or experience
  Synonyms: raw, new