Lois Keith, in the introduction to Musn't Grumble: Writing by Disabled Women (The Women's Press, London, 1993), explains that the title:

. . . came from a piece by Roz Rushworth and is the name of a drama group of disabled women in Bradford she belongs to . . . "Musn't grumble" is what women say to each other when what they really want to do is have a good moan about the things which make them feel fed up . . . it's also a way of shutting up people who assume that our lives are miserable but actually know nothing about us. The kind of people who say "What's wrong with you then?" or "Are you going to get better?" It's what she says to people when what she really means is "Mind your own business". (p8)