What does thus ever to tyrants mean?

"Thus ever to tyrants!" is the English translation of "Sic semper tyrannis!". John Wilkes Boothe, a famous actor in antebellum America and rabid Southern sympathizer, shouted this from the stage where he landed after shooting Abraham Lincoln at Ford's Theater shortly after the end of the Civil War.

The quote is also found on the state flag of Virginia. It was originally attributed to Brutus, one of the assassins of Julius Caesar.