笑不露齒

Idiom

 * 1) to not show one's teeth when one smiles

Usage notes
This is one of the traditional admonishments for Chinese girls, who are instructed that one of female virtues is to not smile easily and to not show their teeth when they smile. Laughing out loud, or any strong display of emotion, is considered ungraceful and unladylike in traditional Chinese culture.

Often used with.