The debate became so heated that they began to lose sight of the original question they were trying to answer.

lose 是动词，意思是"失去、丧失"。 sight 是名词，本义是"视力、视觉"，引申为"看见的事物"或"视野"。 of 是介词，表示所属关系或关联。 lose sight of 是一个固定短语，字面意思是"失去对...的视线"，引申为"忘记、忽视"某个重要的事情或目标。