1. not proven or supported by evidence
Something which is unsubstantiated is not supported by facts or not proven to be true. Something described as being 'unsubstantiated' might just be a rumour or a claim made without any evidence to back it up. Unsubstantiated is similar in meaning to 'unconfirmed' 'uncorroborated' or 'unsupported'. This is an adjective used often in academic and legal contexts.