The phrase means ‘young and without much experience, naive.’
The allusion is to the inexperience of a baby, so recently born as to still be wet.
Another interpretation is that it derives from a mild mockery of children who forget to dry the area behind the ears after a bath. The assumption is that adults, being more experienced, would know better.
Since the idiom originated in America, it’s also been suggested that “wet behind the ears” started off as a description of newborn calves, which begin their lives covered in mucus and other fluids. The last part of their bodies to become completely dry are patches located behind their ears. In that sense, the phrase was cowboy shorthand for an inexperienced or green ranch hand.
So take your pick!
This phrase was in circulation in the USA in the 19th and early 20th century. The converse of the phrase, ‘dry back of the ears,’ was also known in the USA from around the same time. But perhaps not in Canada, for I’ve never heard this one used.
To be wet behind the ears essentially means to be inexperienced, unseasoned or even a bit naïve or immature. In the business world, this phrase is often applied to new employees who are not quite ready to take on the full responsibility of their positions. It’s also common for older people to express skepticism over a young person’s skills or level of authority by claiming he or she is still “wet behind the ears.”
And, until a person reaches a certain level of maturity or gains a certain amount of “street smarts,” he or she could still be considered a little wet behind the ears.
I’ve left childhood far behind, but I still come across circumstances when I feel wet behind the ears rather than dry. For example, I’m about to go do my income tax return and my ears are fairly dripping!