wet behind the ears

The story

1. German · calque offeucht hinter den Ohren
2. English todaywet behind the ears

First use appears c. 1850 in Pennsylvania, a calque of German feucht hinter den Ohren. From the drying of amniotic fluid of a baby after birth, specifically a newborn farm animal, whose ears are the last to dry, partly because it is licked dry by its mother everywhere else. Alternative forms also derive from German.