IPA: /ˈɛksərˌsaɪzər/
KK: /ˈɛksərˌsaɪzər/
A person or device that engages in physical activity to improve fitness or health.
She is an exerciser who goes to the gym every day to stay fit.
Exerciser → It is formed from "exercere" (meaning to keep busy or to train) and "-er" (meaning a person who). An exerciser is a person who keeps busy with physical training or exercise.
Think of someone who is 'keeping busy' ('exercere') with physical activity — that's why an exerciser is a person who engages in exercise.