Fig.: to keep quiet; to refrain from speaking before you say something you shouldn't

I knew he was up to something and I had to bite my tongue not to let it out

Sapevo che stava tramando qualcosa e ho dovuto morsicarmi la lingua per non lasciarmelo sfuggire

My mother-in-law watches all the cooking TV shows and thinks she's a great cook herself. Not a chance! I often have to bite my tongue not to tell her what I really think of her cooking

Mia suocera segue tutti I programmi TV di cucina e crede di essere una brava cuoca. Nemmeno per idea! Spesso devo mordermi la lingua per non dirle quello che penso realmente della sua cucina

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.