The phrase "Et tu, Brute?" which translates to "Even you, Brutus?" was written by William Shakespeare. It was one of the last lines uttered by the title character of his play "Julius Caesar." Because of the circumstances in which the line was uttered in the play, the expression is still used in modern times to express shock at the betrayal of a friend.
Julius Caesar was a dictator of Rome who was murdered by a group of conspiring senators. Caesar's friend Marcus Brutus was part of the conspiracy. In the Shakespearean play, Caesar was stabbed once by each of his attackers. Accounts from the time suggest that he resisted at first, until his friend Brutus appeared and stabbed him too, at which point he gave in to the attack.
The character of Caesar's final words are, "Et tu, Brute? Then fall Caesar!" The betrayal is all the more surprising to Caesar because of his friendship with Brutus and Brutus' reputation for honor. The first line conveys Caesar's shock and disappointment. The second line, Caesar's acceptance of death, is sorrowful and resigned. He cannot bear to survive the pain of Brutus's betrayal.
The phrase is highly popular, and is often quoted in situations where betrayal comes from an unexpected source, especially if the betrayer is a friend.