'unofficially' originates from the prefix 'un-' meaning 'not' and the word 'officially', which comes from Latin 'officialis', meaning 'pertaining to duty or service'.

'officialis' transformed into the Old French word 'official', and eventually became the modern English word 'official'. The prefix 'un-' was added to form 'unofficially'.

Initially, 'officialis' meant 'pertaining to duty', but over time, 'officially' evolved to mean 'authorized or recognized', and 'unofficially' as its negation.