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 accept the full responsibility of their positions yet. It is also common for older people to express skepticism over a younger person's skills or level of authority by claiming he or she is still "wet behind the ears." At some point, however, a rookie or trainee usually does gain the needed experience or wisdom.
The origin of the phrase appears to be purely American, with some sources tracing it back several hundred years. It is more likely that the description came into popular use around the 19th century, as the lingo of the cowboys became more common. One theory holds that "wet behind the ears" started off as a description of newborn and young calves, which began their lives covered in mucus and other fluids. As these calves matured, the last part of their bodies to become completely dry were patches located behind their ears. In that sense, the phrase was cowboy shorthand for an inexperienced or green ranch hand.
It is also possible that the phrase "wet behind the ears" started with human babies, who are also born covered in a layer of mucus and tissue. Unlike a calf, however, a human baby is often dried by a nurse or its mother shortly after birth. The analogy could be more metaphorical in nature, however. Until a child 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.
This is not to say that being considered "wet behind the ears" is necessarily a bad thing. Virtually every occupation or interest has a breaking-in period, and eventually most people achieve a state of competency and maturity as their skills improve. Almost everyone experiences a feeling of being unsure of themselves when confronted with a more advanced set of responsibilities or job requirements. Time and experience are the only things that will help convert a young intern or student into a competent and confident employee.
