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.

Definitions of disgruntled

adj in a state of sulky dissatisfaction

discontent, discontented
showing or experiencing dissatisfaction or restless longing

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