Wiktionary
be mum
vb. (alternative form of be mother English)
Usage examples of "be mum".
But he promised to be mum, and, as he said, there was no need for my name to be mentioned at all in the business, at all events at present, and perhaps not at all, as I was not at the bungalow when Searle was there but at the mess, luckily for me!