Fig.: don't say anything!

I know very well that your mother-in-law is a nosy busybody but, for your family's peace, don't say anything, bite your tongue and let it go

So benissimo che tua suocera è una impicciona ficcanaso, ma, per la pace in famiglia, non dire niente, morditi la lingua e lascia perdere

This expression refers to the effort of refraining from saying something one may regret by literally holding the tongue between the teeth. Shakespeare used it Henry VI, part 2 (1:1):
Ready to starve and dare not touch his own:
So York must sit and fret and bite his tongue,

While his own lands are bargain'd for and sold.