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1.
adj. having the potential to ruin the faces off those who oppose you. an action or circumstance marked by a sense of despair. an object that is highly desirable, to the point of causing extreme jealousy in others.
"dood, back up... i got a ruinous backhand and you're testing my limits"

"in a ruinous turn of events, Ryan's dad decided to accompany the young couple on their camping trip."

"omg man... this chocolate cake you made... it's effin ruinous"
by tehdubbid May 30, 2006
 
2.
(adjective) destructive or harmful.
Man, it's ruinous to have a thing for this guy. Chicks don't know what they're really up for. He is the most sacrosanct guy I've met and honest about himself. But, I know he muff's his friends women and leaves them.
by Accedntalyn<3 December 12, 2009