The Tarpeian Rock was a cliff-edge of the Capitol in Rome, from which murderers and traitors were hurled. As an aetiology for this, early Roman historians told the story of Tarpeia, a woman who featured (as either traitor or heroine) in the war between Romulus and Titius Tatius; cf. under sigma 1757.
See further OCD(4) s.v. "Tarpeia" and "Tarpeian Rock".