Disgruntled sounds like what it is — dissatisfied, grunting and grumbling. You could become a disgruntled employee if your boss swipes all your best ideas without giving you credit (or a raise).
Disgruntled actually comes from gruntle, an old verb meaning, not so surprisingly, "to grunt." When you're disgruntled, you might grunt with dissatisfaction and anger. If you are a disgruntled customer, why not ask to speak to a manager? Unless you'd rather just stand there grunting.
