The Collaborative International Dictionary
Ruinous \Ru"in*ous\, a. [L. ruinosus: cf. F. ruineux. See Ruin.]
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Causing, or tending to cause, ruin; destructive; baneful; pernicious; as, a ruinous project.
After a night of storm so ruinous.
--Milton. Characterized by ruin; ruined; dilapidated; as, an edifice, bridge, or wall in a ruinous state.
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Composed of, or consisting in, ruins.
Behold, Damascus . . . shall be a ruinous heap.
--Isa. xvii. 1.Syn: Dilapidated; decayed; demolished; pernicious; destructive; baneful; wasteful; mischievous. [1913 Webster] -- Ru"in*ous*ly, adv. -- Ru"in*ous*ness, n.
Wiktionary
n. The quality of being ruinous.