wet behind the ears, (still)

Immature, inexperienced. This term refers to the fact that the last place to dry on a newborn colt or calf is the indentation behind its ears. Although the observation is surely older, the term dates from the early twentieth century. J. F. Straker used it in his novel A Coil of Rope (1962): “You’re still wet behind the ears, darling. It’s time you grew up.”