'ruinous' originates from Latin, specifically the word 'ruinosus,' where 'ruina' meant 'collapse or downfall.'

'ruinosus' transformed into the Old French word 'ruinos,' and eventually became the modern English word 'ruinous' through Middle English.

Initially, it meant 'prone to collapse or destruction,' but over time it evolved into its current meaning of 'causing destruction or being excessively costly.'