Defect

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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’.

wiktionary

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From Middle French defaicte, from Latin defectus(“a failure, lack”), from deficere(“to fail, lack, literally 'undo'”), from past participle defectus, from de-(“priv.”) + facere(“to do”).

etymonline

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defect (n.)

early 15c., "want or lack of anything," especially lack of something essential to perfection or completeness, from Old French defect and directly from Latin defectus "failure, revolt, falling away," noun use of past participle of deficere "to fail, desert," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put").

defect (v.)

1570s, "to hurt, damage;" 1580s, "fail become deficient" (senses now obsolete); 1590s, "to desert, revolt," from Latin defectus, past participle of deficere "to fail, desert," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put"). Related: Defected; defecting.