To keep one's feelings to oneself; to show neither joy nor anger; to be composed and reserved; to maintain a poker face.
无论遇到多大的困难，他总是喜怒不形于色，沉着应对，展现出过人的心理素质。
No matter how great the difficulties he encountered, he always kept his feelings to himself and responded with composure, demonstrating extraordinary psychological fortitude.
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