IPA: //ˈbækstroʊk//
KK: /ˈbækˌstroʊk/
A swimming style where a person lies on their back and moves their arms in a circular motion while kicking their legs to propel themselves through the water.
She won the gold medal in the backstroke event at the swimming competition.
Backstroke is formed from "back" (from Old English "bæc", meaning the rear or reverse side) and "stroke" (from Old English "strōc", meaning a movement or action). The word refers to a swimming style where the swimmer lies on their back and uses a specific arm movement to propel themselves through the water.
Think of swimming on your 'back' — that's why 'backstroke' describes this particular swimming style.
No commonly confused words.