The frase "Empty Barrel" first found in the writing a Plato later used by Shakespeare refers to a person who makes a lot of noise but has no substance.
An empty barrel makes the most noise when it is knocked. When you call someone an "empty barrel" you are calling them stupid or a moron, as in nothing in your head.
A sly, euphemistic way of saying "uppity nigger" - without seeming like an obvious bigot. Also used in misogynistic ways to describe vocal black women by patronizing, old, white assholes.