'proclivity' originates from Latin, specifically the word 'proclivitas', where 'pro-' meant 'forward' and 'clivus' meant 'slope' or 'leaning'.

'proclivity' changed from the Late Latin word 'proclivitas' and entered English (via scholarly/learned borrowing) as the modern English word 'proclivity' in the 17th century.

Initially, it meant 'a leaning or slope' (both literal and figurative), but over time it evolved into its current meaning of 'a natural inclination or tendency,' often used figuratively for behavioral tendencies.