grouch

This term carries a colloquial, slightly playful tone, often used to describe someone whose irritability is seen as a personality trait rather than a temporary mood. It suggests a persistent, low-level dissatisfaction that is more annoying than truly aggressive, often evoking the image of a person muttering under their breath.

When used as a verb, it emphasizes the act of complaining without a constructive goal. It differs from "complain" by focusing on the grumpy attitude of the speaker rather than the validity of the grievance.