'someday' originates from Middle English, specifically the combination of 'some' and 'day', where 'some' meant 'a certain' and 'day' referred to a '24-hour period'.

'someday' changed from the Middle English phrase 'someday' and eventually became the modern English word 'someday'.

Initially, it meant 'at some unspecified time', and this meaning has largely remained the same in modern usage.