hold one's tongue
Also, hold or keep one's peace. Keep quiet, remain silent, as in If you don't hold your tongue you'll have to go outside, or Jenny kept her peace about the wedding. The idiom with tongue uses hold in the sense of "restrain," while the others use hold and keep in the sense of "preserve." Chaucer used the first idiom in The Tale of Melibus (c. 1387): "Thee is better hold thy tongue still, than for to speak." The variant appears in the traditional wedding service, telling anyone who knows that a marriage should not take place to "speak now or forever hold your peace." [First half of 1300s] Also see keep quiet.