Et tu, Brute? is Latin and, in common translation, means "and you too, Brutus?": it is what Caesar says in the play when he sees Brutus among those who stab him, in Act III, scene 1, just before saying "Then fall, Caesar!" and dying.
In the play, it is Antony who voices the weight of this moment. In the Forum (Act III, scene 2), he says that Brutus was Caesar's angel and that this was the cruelest of the blows: ingratitude, stronger than the arms of the traitors, overcame him.
The Latin phrase, however, does not come from the ancient historians. Suetonius reports that Caesar gave only a groan, without a word, at the first blow, although some have related that he said to Brutus, in Greek, "You too, my son?", which connects to the old rumor that Brutus was Caesar's natural son. Plutarch records no words at all: he says that, according to some authors, when Caesar saw Brutus with a dagger in hand, he pulled his toga over his head. "Et tu, Brute?" is a Latin formula consecrated by tradition, and not an account of what was said.
The play itself preserves this second version of the scene. In the Forum, Antony recounts that Caesar, seeing Brutus strike him, covered his face with his cloak and fell at the foot of Pompey's statue.