'unsubstantiated' originates from the prefix 'un-' meaning 'not' and the word 'substantiated' from Latin 'substantiatus', where 'substantia' meant 'substance'.

'substantiatus' transformed into the English word 'substantiated', and with the addition of the prefix 'un-', it became 'unsubstantiated'.

Initially, it meant 'not having substance or evidence', and this meaning has largely remained the same in modern usage.