He was under the misapprehension that the meeting was tomorrow, but it's actually this afternoon.

misapprehension 是由前缀 mis-（表示"错误的"）和名词 apprehension（理解、领会）组成的复合词。apprehension 来自动词 apprehend（理解、领会）。 misapprehension 的字面意思是"错误的理解"，即"误解、误会"。