'unusual' originates from the Latin word 'usualis,' where 'un-' is a prefix meaning 'not' and 'usualis' meant 'customary or habitual.'

'usualis' transformed into the Old French word 'usuel,' and eventually became the modern English word 'usual' through Middle English. The prefix 'un-' was added to form 'unusual.'

Initially, 'unusual' meant 'not customary,' and this meaning has largely remained the same in modern usage.