cross one's heart指的是在胸口画十字的动作，表示一定会实现诺言或誓言。所以“Cross your heart”意思就是“你发誓”。

【例句】
Promise me you will not tell him what I have said. Cross your heart and hope to die.
答应我，你绝不会把我所说的告诉他。你发誓说了不得好死。

Unless you cross your heart, I won't believe you.
除非你发誓，否则我不信。