, It means just what it says, and is used, for example, when a small child says they have tummy ache and need the day off school, when they don't really. It's a very common saying in the UK. I guess that says a lot about our culture, that we're all trying to white-lie our way through life.
Thank you Brian. I lived in England for some 10 years some 70 years ago but I never came across that saying but one of my daughters (a child then) would often claim a tummy ache so as not to go to school. Needless to say....she had to go.