Having many defects; full of faults or imperfections; inclined to be faulty or defective.
From Latin 'defectus' plus English suffix '-ious' (full of, characterized by). The word is archaic or very rarely used in modern English.
This adjective is practically extinct in English—'defective' replaced it centuries ago—but 'defectious' appears occasionally in old texts, showing how English language evolution sometimes just picks one word over a synonym and lets the other fade away.
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