E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Prophesy upon Velvet (To).
To prophesy what is already a known fact. Thus, the issue of a battle flashed to an individual may, by some chance, get to the knowledge of a sibyl, who may securely prophesy the issue to others; but such a prediction would be a prophecy on velvet; it goes on velvet slippers without fear of stumbling.
If one of those three had spoken the news over again the old lady [or sibyl] prophesies upon velvet.Sir W. Scott: The Pirate, ch. xxi.