Something ruinous is really terrible or devastating, like a ruinous hurricane that floods your house or the ruinous effect of a new law on a country's poorest citizens.

The adjective ruinous perfectly describes something so bad that it truly causes ruin, "the destruction or disintegration of something." A house fire can be ruinous, and so can a bad hand at a poker table, if the player has bet her last dollar. Ruinous and ruin comes from the Latin root ruina, "a collapse, a rushing down, a tumbling down."

Definitions of ruinous

adj extremely harmful; bringing physical or financial ruin

“a ruinous course of action”
causing or capable of causing harm

adj causing injury or blight; especially affecting with sudden violence or plague or ruin

“a ruinous war”
causing destruction or much damage

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