IPA: //ˈbækˌstroʊkər//
KK: /ˈbækˌstroʊkər/
A person who swims using the backstroke technique, lying on their back in the water and moving their arms and legs to propel themselves.
The backstroker won the gold medal in the swimming competition.
Backstroker → It is formed from "back" (from Old English *bæce*, meaning the rear or reverse side) and "stroker" (from the verb "stroke", meaning to move one's arms in a specific manner). A backstroker is someone who swims on their back, using a specific arm movement.
Think of someone swimming on their 'back' — that's why a backstroker is a swimmer who uses their back to float and stroke.
No commonly confused words.