Remain silent, especially so as not to reveal a secret.
‘he was keeping mum about a possible move to West Ham’
‘Think what you may; keeping mum gives me one secret more.’
‘A good friend can keep mum about your deepest darkest secrets.’
‘The CIA though has conveniently kept mum as of now.’
‘The fish sign took two people to make - you made a curved line casually on the ground with a toe, if the other person didn't add the other curve you kept mum.’
‘I kept mum about the fact I'd had an abortion.’
‘I presume that the teachers and administration kept mum and can only suppose that it was the boy himself who publicized his problems.’
‘The hospital authorities also kept mum about his visit.’
‘But New York Times staffers kept mum not so much out of fear of reprisals as out of respect for the institution.’
‘Defying the age of celebrity, and resisting the lucrative market for antiquities, the property owner kept mum about his treasure for decades.’
‘Many Gandhians who had kept mum during earlier riots are protesting this time and actively working for restoration of peace.’