'defect' originates from Latin, specifically the word 'deficere' (past participle 'defectus'), where 'de-' meant 'away/from' and 'ficere' (from 'facere') meant 'to do/make' (sense: to fail or to be lacking).

'defect' came into Middle English via Old French/Anglo-French (e.g. 'defect') from Latin 'defectus'/'deficere' and developed into the modern English forms 'defect' (noun/verb) and derivatives like 'defection'.

Initially, it meant 'to fail' or 'to be lacking' in Latin; over time English developed both the sense of 'a shortcoming' (noun) and the sense 'to abandon or desert' (verb), the latter relating to 'failing in duty' and eventually to leaving allegiance.