'additionary' originates from Latin, specifically the word 'additio', where 'add-' meant 'toward' and '-itio' meant 'action or process'.

'additio' transformed into the French word 'addition', and eventually became the modern English word 'additionary' through Middle English.

Initially, it meant 'the action of adding something', but over time it evolved into its current meaning of 'relating to addition'.