General synonyms:
* Grumpy: Implies a state of persistent bad mood and irritation.
* Cranky: Similar to grumpy, but often with a sense of being easily upset.
* Surly: Implies a hostile or bad-tempered attitude.
* Sullen: Indicates a gloomy or withdrawn mood.
* Cantankerous: A strong term for someone who is argumentative and bad-tempered.
* Peevish: Describes someone who is easily annoyed and complains frequently.
Synonyms emphasizing mumbling:
* Grouch: A person who often complains and is in a bad mood.
* Gripe: To complain in a nagging way.
* Whine: To complain in a feeble or high-pitched voice.
* Mutter: To speak in a low, indistinct voice.
More specific synonyms:
* Curmudgeonly: A humorous term for a grumpy, bad-tempered old person.
* Petulant: Describes someone who is sulky and bad-tempered, especially over a trivial matter.
The best synonym will depend on the specific context and the tone you want to convey.