"Unsubstantiated rumours" refers to stories or information that have not been proven or supported by evidence. It means there is no proof or factual basis for the rumours being spread.
Full definition
Seeing their newspaper and TV audiences haemorrhaging last summer the main stream media empires that run the national papers started to set up bloggettas of their own — and not just one each, but in some cases a dozen such sites, each recycling the wild and
unsubstantiated rumours of each other, each quoting the other as the source.
I've heard one
unsubstantiated rumour about the RS5's exploits around the Nürburgring Nordschleife (sub 8 minutes), and if that proves correct it would seem Audi have got it right.
It is therefore highly unlikely that a sober person will come up with a highly negative article based on
an unsubstantiated rumour.
My immediate supervisor began spreading, to managers higher up,
unsubstantiated rumours that my research work was not up to standard and thereby nearly prevented me from attending science - related seminars and necessary training.
These are
unsubstantiated rumours and nothing more.
For now, it's nothing more than
an unsubstantiated rumour, but it's an interesting prospect in the run up to the Apple Watch launch.