The police arrested a wiretapper who tried to eavesdrop on government officials' calls.

wiretapper 是由 wire（电线）和 tap（轻敲、窃听）加上后缀 -er（表示人）组成的复合名词。wiretap 本意是"在电话线上搭线窃听"，wiretapper 指"进行电话窃听的人"，即"窃听者"。