He was caught by customs for carrying undeclared cash when entering the country.

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