What's the idiomatic way of saying "hide your lips"? I thought "bite your lips", but you're not biting your lips, your putting your lips inside your mouth. So what's the proper way to say it? I added a pic of someone doing it.
Lambie's answer describes the action of curling one's lips inwards. But if the OP is looking for a shorter expression, then I would say that the young mother in the photo appears tight-lipped.
1 : having the lips closed tight (as in determination)
2 : reluctant to speak : TACITURN
Cambridge Dictionaries has
Someone who is tight-lipped is pressing their lips together to avoid showing anger, or is refusing to speak about something: