IPA: /ˈwaɪərˌtæpər/
KK: /ˈwaɪərˌtæpər/
A person who secretly listens to or records conversations on a telephone or other communication device without the knowledge of the people involved.
The police used a wiretapper to gather evidence in the investigation.
Wiretapper → It is formed from "wire" (from Old English *wīr*, meaning a slender, flexible strand) and "tapper" (from the verb "tap", meaning to strike lightly or to draw off). The word "wiretapper" refers to someone who strikes or draws off information from a wire, typically in a clandestine manner.
Think of a 'slender strand' ('wire') being 'tapped' for information — that's how a wiretapper secretly gathers data.