keep a still tongue (in one's head)

To remain silent, especially until it is prudent or one has thought carefully about what one wants or needs to say; to not speak out of turn. I felt anger surging up in me after her caustic comments, but I decided to keep a still tongue and not say something I would later regret. If I may offer some advice, you should learn to keep a still tongue in your head instead discussing such personal affairs so openly in mixed company.