Where does the saying “not dry behind the ears” come from and What does it mean?

The expression “not dry behind the ears” means as innocent and unsophisticated as a babe.

A saying that came directly from the farm, where many others have also arisen, for it alludes to a newly born animal, as a colt or a calf, on which the last spot to become dry after birth is the little depression behind either ear.

The figurative use seems to be wholly American, too homely to have attained literary pretensions, but undoubtedly in familiar use through the past hundred years or longer.