During the debate, the politician lost his cool and insulted his opponent.

lost 是动词 lose 的过去式，lose 的意思是"失去、丧失"。 his 是代词 he 的所有格形式，意思是"他的"。 cool 在这里是名词，意思是"冷静、镇定"。 lost his cool 是一个固定短语，字面意思是"失去了他的冷静"，实际表示"失去冷静、发脾气"的意思。