Wet behind the ears
The allusion is to the inexperience of a baby, so recently born as to be still wet.
This phrase was in circulation in the USA in the early 20th century - twenty years before it was first recorded elsewhere. The converse of the phrase - 'dry back of the ears', was also known in the USA from around the same date. That was recorded in the American Dialect Society's Dialect Notes IV, 1914:
"Dry back of the ears, mature; - of persons."
The earliest citation I can find for 'wet behind the ears' is from the Portsmouth Daily Times, October 1911:
"There is not much in the matter so far as the organ [the courthouse record] is concerned except it is so new that it is wet behind the ears yet".