Fig.: don't say anything!

Our new boss has a shitty temper and when he's angry, he's impossible to deal with, so even if you disagree with him, hold your tongue and don't say anything until he has calmed down

Il nuovo capo ha un carattere di merda e quando è arrabbiato è intrattabile, perciò, anche se non sei d'accordo con lui, frena la lingua e non dirgli niente finché non si è calmato

To hold is used in the sense of “refrain”, so if you refrain your tongue, you keep it from talking.
Geoffrey Chaucer used this idiom in The Tale of Melibee, one of the 24 stories narrated in The Canterbury Tales:
This is to seyn, that thee is bettre holde thy tonge stille than for to speke
(This is to say, that for thee it is better hold thy tongue still than to speak)