You can say ' cross my heart' when you want someone to believe that you are telling the truth. You can also ask 'cross your heart?', when you are asking someone if they are really telling the truth. The above answer is from google. Thanks for adding a new phrase to my dictionary.

Hello Shahrzad, It means I promise I'm telling the truth. معمولا واسه وقتی استفاده میشه که یه نفر اسرار داره و قول میده که داره حقیقت رو میگه.

I dont know

You try to persuade people what you say is true real. Okay.I got you.Next time I won't say I vow,I promise,I swear any more.I'll say Cross my heart.

I promise, I swear. I told the truth, the whole truth, nothing but the truth.