'defector' originates from Latin, specifically the word 'deficere' where 'de-' meant 'away/from' and 'ficere' (from 'facere') meant 'to do/make'; in the compound 'deficere' it meant 'to fail' or 'to withdraw'.

'defector' developed from Latin 'deficere' and Late Latin/Medieval Latin forms meaning 'one who withdraws' and passed into English via Old French/Medieval Latin formations to become the modern English 'defector'.

Initially it meant 'one who withdraws or is lacking'; over time it came to mean specifically 'a person who abandons allegiance to a country, party, or cause' (often politically or militarily).