ROGET THESAURUS
keep mum
Taciturnity
VB
be silent, keep silence,
keep mum, hold one's tongue, hold one's peace, hold one's jaw, not speak, say nothing, keep one's counsel, seal the lips, close the lips, button the lips, zipper the lips, put a padlock on the lips, put a padlock on the mouth, put a bridle on one's tongue, bite one's tongue, keep one's tongue between one's teeth, make no sign, not let a word escape one, keep a secret, not have a word to say, hush up, hush, lay the finger on the lips, place the finger on the lips, render mute, stick in one's throat.