'addn' originates from the English word 'addition', which comes from the Latin word 'additio', where 'ad-' meant 'toward' and 'ditio' meant 'giving'.

'additio' transformed into the Old French word 'addicion', and eventually became the modern English word 'addition', abbreviated as 'addn'.

Initially, it meant 'the act of adding something', and this meaning has largely remained the same in modern usage.