He had a vague sense of what the lecture was about but couldn't articulate any details.

vague 是形容词，意思是"模糊的、不明确的"，描述一种不清楚或不确定的状态。这个词来自拉丁语 vagus，原意是"漫游的、不固定的"，引申为"不明确的"。