Definnition: Young or inexperienced.

Origin
I’ve never personally observed this, but supposedly when animals are born they tend to be a soggy mess. The last place that finally becomes dry is behind the ears. Congratulations to lori_simms for being the first pig player to come up with the correct answer.

Answers White Elephant Don't look a gift horse in the mouth Wet behind the ears Raining Cats and Dogs I'll be a monkey's uncle