He carried a perfectly unsuspicious briefcase that didn't attract any attention from the guards.

unsuspicious 是由前缀 un-（表示"不、非"）和形容词 suspicious（可疑的）组成的形容词。suspicious 来自动词 suspect（怀疑），所以 unsuspicious 的字面意思是"不可疑的"，引申为"看起来普通的、不引人注意的"。