Mustn't grumble is a very British way of admitting things are all right. Mustn't grumble, I suppose, well, mustn't grumble.
It concedes that life is bearable while refusing to say so directly. It belongs to older speakers and to a whole tradition of answering a kind question with a shrug.
1phrasemustn't grumbleA very British way of admitting things are all right.
“He looked at the weather and said mustn't grumble.”
mustn't grumble is a phrase.
Practised in Small Talk & Everyday Phrases