"Crossing your heart" means to make the sign of the crucifix on your body by pointing to the four ends of the cross in turn. It is a Christian gesture (usually Catholic) intended to suggest that you are making an oath before God. In common use now, it is an idiom meaning that you are making a solemn promise or telling the truth, from the child's rhyme: "I cross my heart and hope to die, should I ever tell a lie."
So if someone says to you that they never drink alcohol, for example, you might reply by asking "Cross your heart?" which means "Do you solemnly swear that is the truth?"
- For Teachers