His instructions were too vague, so we didn't know what to do exactly.

vague 是形容词，意思是"模糊的、不明确的"，描述事物不清楚或难以理解的状态。这个词来自法语 vague（波浪），引申为"不确定的、含糊的"，就像波浪的形状难以捉摸一样。