'propensity' originates from Latin, specifically the word 'propensus', from the past participle of 'propendere', where 'pro-' meant 'forward' and 'pendere' meant 'to hang (or weigh)'.

'propensity' changed from the Middle French word 'propensité' (and Middle English forms) and eventually became the modern English word 'propensity'.

Initially, it meant 'a leaning or inclination' (literally 'leaning forward'), but over time it evolved into its current meaning of 'a natural inclination or tendency'.