IPA: /ˈæmˌbʌʃər/
KK: /ˈæmˌbʌʃər/
A person who attacks someone unexpectedly from a hidden place.
The ambusher waited silently in the bushes for the right moment to strike.
Ambusher → It is formed from "ambush" (from Old French *embuschier*, meaning to hide in a thicket) and "-er" (meaning a person who). An ambusher is a person who hides in order to attack unexpectedly.
Think of someone who 'hides' ('ambush') to 'attack' — that's why an ambusher is a person who attacks from a hidden position.