[bite] {one's} lip

to press one's teeth against the lip as a reaction to emotion, pain, or to prevent oneself from saying something

She had to bite her lip to prevent herself from saying something hurtful during the argument.
During the intense negotiation, he bit his lips to avoid showing his frustration.
The child bit his lip to keep from crying after falling down.
She bit her lips in anticipation while waiting for the exam results.
Even though she was nervous, she bit her lip and gave a confident speech.