Mustn't grumble
What it really means
A very British answer meaning "life is okay". Grumble means complain. The phrase suggests there are small problems, but nothing worth moaning about. Especially common among older speakers.
Context, tips & common mistakes
"Mustn't grumble" answers 'how are you?' with the national ideal: things are tolerable and I am not complaining about the rest. It sits with 'can't complain', 'not too bad', 'surviving' and 'oh, you know' in the family of deliberately unspectacular answers — British small talk treats both boasting and genuine complaint as impositions, so wellbeing is reported in shrugs.
The joke underneath: Britain complains constantly (weather, trains, prices) while prizing not-complaining as a virtue — the resolution is that recreational grumbling about shared externals is bonding, while grumbling about your own life is burden. Answer 'how are you?' in kind ('not bad at all, you?'), save real news for real conversations, and read 'mustn't grumble' from others as contentment unless the tone says otherwise.