A spurious Egyptian monument sold by a soldier to Cardinal Bembo in 1527, and preserved at Turin. It is of copper, and on it are represented most of the Egyptian deities in the mysteries of Isis. It was said to have been found at the siege of Rome in 1525. The word Isiac is an adjective formed from Isis.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894