A Roman augur in the reign of Tarquin the Elder. When he
forbade the king to increase the number of the tribes without
consulting the augurs, Tarquin asked him if the thought then in his
mind was feasible. “Undoubtedly,” said Accius. “Then cut through this
whetstone with the razor in your hand.” The priest gave a bold cut, and
the block fell in two. This story (from Livy, Bk. i., chap. 36) is
humorously retold in Bon Gaultier's Ballads.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894