'legionary' originates from Latin, specifically the word 'legionarius', where 'legio' meant 'levy, a body of troops' and the suffix '-arius' denoted 'pertaining to or connected with'.

'legionarius' passed into Medieval/Old French and Middle English forms (e.g. Old French/Anglo-Norman influence), and eventually became the modern English word 'legionary'.

Initially, it meant 'pertaining to or a member of a legion', and over time it retained that sense, being used for ancient Roman soldiers and later for members of modern units called legions.