If something is ruinous, it causes great harm or damage.
When something is ruinous, it is very bad and can cause a lot of trouble.
Examples:
The storm was ruinous for the old house.
Eating too much candy is ruinous for your teeth.
The ruinous earthquake destroyed the city.
The ruinous decision cost him a lot of money.
The ruinous fire burned down the whole forest.
The ruinous flood washed away the bridge.