'declivitous' originates from Latin, specifically the word 'declivis,' where 'de-' meant 'down' and 'clivus' meant 'slope.'

'declivis' transformed into the Medieval Latin word 'declivitas,' and eventually became the modern English word 'declivitous.'

Initially, it meant 'having a downward slope,' and this meaning has largely remained the same in modern usage.