(the younger) being banished a second time from Syracuse, retired to Corinth, where he turned schoolmaster for a living. Posterity called him a tyrant. Byron, in his Ode to Napoleon, alludes to these facts in the following lines—
Corinth's pedagogue hath now Transferred his byword to thy brow.
That is, Napoleon is now called tyrant like Dionysius.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Dionysius from Fact Monster: