IPA: /ˈkɪdˌnæpər/
KK: /ˈkɪdˌnæpər/
A person who unlawfully seizes and carries away a person by force or fraud, often for ransom or other demands.
The police are searching for the kidnapper who took the child last week.
Kidnapper is formed from "kid" (from Middle English *kidde*, meaning a young goat or child) and "nap" (from the Old French *naper*, meaning to seize or take). The word refers to someone who seizes or takes away a child.
Think of someone who 'seizes' ('nap') a 'child' ('kid') — that's why a kidnapper is someone who takes a child away.