bite one's tongue
Refrain from speaking out, as in A new grandmother must learn to bite her tongue so as not to give unwanted advice, or I'm sure it'll rain during graduation.Bite your tongue! This term alludes to holding the tongue between the teeth in an effort not to say something one might regret. Shakespeare used it in 2 Henry VI (1:1): "So York must sit and fret and bite his tongue." Today it is sometimes used as a humorous imperative, as in the second example, with the implication that speaking might bring bad luck. [Late 1500s] Also see hold one's tongue.
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