'mountainous' originates from Middle English, specifically the word 'mountainous,' where 'mountain' meant 'a large natural elevation of the earth's surface.'

'mountainous' changed from the Old French word 'montaineux' and eventually became the modern English word 'mountainous'.

Initially, it meant 'full of mountains,' but over time it evolved into its current meaning of 'having many mountains or resembling a mountain in size.'