bite (one's) lip

To forcibly prevent oneself from exhibiting an outward reaction to something, especially that which makes one angry, irritated, or upset. I just had to bite my lip while the boss yelled at me for losing the account. If someone tries to provoke you, just bite your lip and keep walking. I had to bite my lip as my sister gushed about her new boyfriend yet again.

bite your lip

repress an emotion; stifle laughter or a retort.

bite your ˈlip

force yourself not to express the negative emotions that you are feeling: You could tell she thought the criticism was unfair but she bit her lip and said nothing. OPPOSITE: let rip (at somebody) (with something)