Wiktionary
hold one's tongue
vb. (context idiomatic English) to keep quiet; especially, to leave something unsaid
Usage examples of "hold one's tongue".
When one gets the youth of England on the subject of its Scarlet Pimpernel, one can only smile and hold one's tongue.
If after the help one has had from you one can't either take care of one's self or simply hold one's tongue, one must renounce all claim to be an object of interest.