Bite your tongue.
Candid street shot St Austell Cornwall.
In order to stop yourself saying something that you would really like to say.
When a person wants to say something that may be offensive or hurtful to someone, so instead says nothing, as opposite of speaking their mind.
The notion of biting one's tongue instead of speaking or taking action goes back (in English) at least to Shakespeare. From Henry VI Part 2 (1591):
"Pirates may make cheap penn'worths of their pillage, And purchase friends, and give to curtezans, Still reveling, like lords, till all be gone: While as the silly owner of the goods Weeps over them, and wrings his hapless hands, And shakes his head, and trembling stands aloof, While all is shar'd, and all is borne away; Ready to starve, and dares not touch his own. So York must sit, and fret, and bite his tongue, While his own lands are bargain'd for, and sold".