To keep a stiff upper lip means to remain courageous and composed, especially in the face of adversity or difficult situations. It implies a refusal to show emotion or complaint.

For example:

Despite the bad news, she managed to keep a stiff upper lip and carry on.

He was taught from a young age to keep a stiff upper lip and not show his feelings.

The soldiers were instructed to keep a stiff upper lip even when facing enemy fire. 

Even though he was heartbroken, he forced himself to keep a stiff upper lip. 

The team lost the championship game, but they all kept a stiff upper lip and congratulated the winners. 

Tradução: não se abalar, não baixar a cabeça, manter-se firme, manter a cabeça erguida,  não deixar transparecer as emoções, manter a pose.