Wet behind the ears

What's the meaning of the phrase 'Wet behind the ears'?

Naive.

What's the origin of the phrase '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".