We were having so much fun that we lost track of time.

lost 是动词 lose 的过去式，意思是"失去、丢失"。 track 是名词，本义是"轨迹、踪迹"，引申为"记录、线索"。 lost track of time 是固定搭配，字面意思是"失去了对时间的追踪"，即"忘记了时间"。