bite (one's) tongue

1. Literally, to accidentally pinch one's tongue with one's teeth. My daughter started crying after she bit her tongue. Gosh, why does it hurt so much when you bite your tongue? Ouch! Yesterday, I bit my tongue so hard it started bleeding. Not fun.

2. To stop oneself from saying something (often something potentially inappropriate, hurtful, or offensive). I had to bite my tongue as my sister gushed about her new boyfriend yet again. I just had to bite my tongue while the boss yelled at me for losing the account. If someone tries to provoke you, just bite your tongue and keep walking.

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.