Fig.: to not give way to adversity; to keep complete control over one's feelings

Although Gregory is about to divorce and lose his house and family, he keeps a stiff upper lip and says that everything is fine

Nonostante Gregorio sia sul punto di divorziare e perdere tutto quello che ama, si mantiene impassibile e dice che tutto va per il meglio

I know you are upset about wrecking your car, but try and keep a stiff upper lip: things will go better

Lo so che sei disperato per aver distrutto la macchina, ma non perderti d'animo: vedrai che le cose cambieranno in meglio

This expression, common from the 1800s, probably refers to a trembling lip, which betrays that one is about to burst into tears. Since it is usually the lower lip that trembles before weeping, keeping the upper lip steady will most likely prevent from crying