Ruinous, a.
Etymology: [L. ruinosus: cf. F. ruineux. See Ruin.]
1. Causing, or tending to cause, ruin; destructive; baneful; pernicious; as, a ruinous project. After a night of storm so ruinous. Milton.
2. Characterized by ruin; ruined; dilapidated; as, an edifice, bridge, or wall in a ruinous state.
3. 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. -- Ru"in*ous*ly, adv. -- Ru"in*ous*ness, n.
Shades of meaning
Fernald has no entry of its own for ruinous; he lists it under pernicious.
He lists it without comment; the entries above explain its neighbors.