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.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894