Causing damage; harmful or destructive; tending to cause injury or loss.
From 'damage' plus the suffix '-ous' meaning 'full of' or 'characterized by'; an archaic or rare adjective form meaning damaging or harmful.
Damageous is essentially a synonym for 'damaging' that almost nobody uses anymore, showing how English constantly prunes away redundant words—we chose 'damaging' over 'damageous' and now the latter sounds like it belongs in a medieval manuscript.
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