ruinruinruins2nouna building that has been badly damaged: The old castle is now a ruin.in ruinsbadly damaged or destroyed: The city was in ruins after the war.ruinruinruinsruinedruining1verb (ruins, ruining, ruined)to damage something badly so that it is no longer good; to destroy something completely: I spilled coffee on my jacket and ruined it. The rain ruined our picnic.