'defection' originates from Latin, specifically the Medieval Latin word 'defectio', ultimately from the Latin verb 'deficere' (to fail, to revolt), where 'de-' meant 'away/from' and 'ficere' (related to 'facere') carried the sense of 'make/do' or was part of the verbal root meaning 'to fail/leave'.

'defectio' in Medieval/ Late Latin passed into Anglo-French/Old French and Middle English (e.g. Middle English 'defeccioun'/'defection'), and eventually became the modern English word 'defection'.

Initially it could mean a failing or lack (a shortcoming), but over time it evolved to primarily denote the act of abandoning allegiance or leaving an organization (the modern sense of 'switching sides' or 'withdrawal').