Suetonius, in his work Vita Divi Iuli, reports the last words of Caesar being Greek καὶ σὺ τέκνον; which is the original source of Shakespeare's line, translated into Latin fairly literally:
- the conjunction καὶ becomes its equivalent et;
- the pronoun σὺ becomes its equivalent tu,
- τέκνον "child" is replaced by M. Iunius Brutus's own cognomen.
An alternative Latin translation, familiar to readers of Astérix, is the similar Tu quoque, fili: here τέκνον is translated by the equivalent filius, but the conjunction et "and, too" becomes quoque "as well, too, even."
In any case, the meaning is simple: Caesar was chastising Brutus for being among the conspirators, that he expected better from him. "Even you, son?" or "You, too, Brutus?" are standard renderings in English. Because of the simplicity of the phrase, it is rather straightforward to interpret and difficult to ascribe variant readings.