effective→効果的な
de + effective→効果がない→欠点のある、不良の
(本当は語源違うみたいですが覚えやすいのでこれで・・・)
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defectの語源
late Middle English (as a noun, influenced by Old French defect ‘deficiency’): from Latin defectus, past participle of deficere ‘desert or fail’, from de- (expressing reversal) + facere ‘do’