English 'fitness' combines the adjective 'fit' (of uncertain origin, appearing in Middle English around the fifteenth century) with '-ness' — originally meaning 'suitability,' it acquired its dominant physical-exercise sense only in the late nineteenth century, with massive amplification in the fitness boom of the 1970s and 1980s.
The condition of being physically strong and healthy; suitability or appropriateness for a particular role or task.
Formed from the adjective 'fit' plus '-ness.' The origin of 'fit' is uncertain and debated. It appears suddenly in Middle English around the fifteenth century with no clear Old English antecedent. Some scholars propose a connection to Middle English 'fitten' (to marshal troops, to array), possibly from Old Norse. Others
Charles Darwin's famous phrase 'survival of the fittest' (coined by Herbert Spencer in 1864, adopted by Darwin in 1869) uses 'fitness' in its original sense of 'suitability' — the best-suited to an environment — not physical strength. The modern gym-culture meaning of 'fitness' has retroactively distorted how most people read Darwin's phrase.