A person who exercises or practices physical activity, or someone who practices a skill or profession.
From exercise + -er suffix, forming an agent noun. The -er suffix is Germanic in origin and is one of the most productive ways to create 'one who does X' words.
The distinction between 'exerciser' (active person) and 'exercisee' (theoretical person being exercised on) rarely occurs, but it reveals how English agent nouns can theoretically pair with patient nouns.
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