Meaning
To bite one's tongue
Example
Tive que morder a língua para não falar o que eu realmente pensava do chefe.
I had to bite my tongue to not say what I really thought about the boss.
Origin
The image is bodily and immediate: to clamp your teeth on your own tongue is to stop it moving and silence the words it was about to form. Morder a língua thus means to hold back something you were on the verge of saying — or, after the fact, to wish you could take it back.
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More idioms
Acertar na moscaTo guess correctly / To hit the nail on the headAmigo da onçaA fake/false friendAndar na linhaTo toe the line / To behave properlyArmar um barracoTo cause a public sceneRodar a baianaTo cause a public scene / To go ballisticArrumar sarna pra se coçarTo ask for troubleArrastar as asasTo hit on someone / To flirtAbrir o jogoTo come clean / To open upArregaçar as mangasTo roll up one's sleeves / To get to workAgarrar com unhas e dentesTo hold on for dear life / To seize an opportunity