She was stopped at customs for carrying undeclared luxury items.

undeclared 是由前缀 un-（表示“未、不”）和动词 declare（申报、声明）的过去分词 declared 组合而成的形容词。un- 表示否定，declared 表示“已申报的”，所以 undeclared 意思是“未申报的”。