Other forms: wetter behind the ears; wettest behind the ears
If someone is wet behind the ears, it means that they're too young and inexperienced to really know their way around a situation.
This term often appears in the form still wet behind the ears, as in "I can't believe they put him in charge of this operation when he's still wet behind the ears." It may refer to the idea that newborn babies are born still soaked with the fluid they were floating in while in the womb, and the area behind the ears takes a while to dry. It's kind of like saying someone was born yesterday, another expression that means "inexperienced and naive."