From the Latin sīc semper tyrannīs (“thus always to tyrants”). While the line is sometimes said to have been uttered by Brutus after he assassinated Julius Caesar, the utterance itself is recorded in no ancient sources and appears to be a modern invention. It is probably a Latin translation by the US Founding Father George Wythe of what Tiberius Gracchus' grandfather, the general and statesman Scipio Aemilianus, said when he heard of the assassination of his grandson. According to Plutarch (21.4), he reacted by quoting Homer's Odyssey (1.47): ὡς ἀπόλοιτο καὶ ἄλλος, ὅτις τοιαῦτά γε ῥέζοι (And so perish all who do the same). This etymology is presented by Mike Fontaine,[1] though he mentions both Wythe and George Mason as possible translators, which is very unlikely since Wythe is famous as a Classicist whereas there is no recorded mention of Mason's knowledge of Greek.
John Wilkes Booth supposedly uttered it when he assassinated Abraham Lincoln. This is probably the most famous use of this expression.