late 14c., "going to ruin," from Old French ruinos (Modern French ruineux) or directly from Latin ruinosus "tumbling down, going to ruin," from ruina (see ruin (n.)). Meaning "causing ruin" is from mid-15c. Related: Ruinously.
a ruinous course of action
a ruinous war
rugby
rugged
rugrat
ruin
ruination
ruinous
rule
ruler
ruling
ruly
rum
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