Cambridge Dictionary.

Cross my heart (and hope to die) is an expression that means to show that what you have just said or promised is completely true or sincere.

For example, I’m deadly serious—cross my heart.

Mom, I’ll clean my room tomorrow. I mean it. Cross my heart!

I didn’t hear anything about Bob’s cheating on Beth-cross my heart.

I didn’t take it, cross my heart!

Also, the full version of the expression is “Cross my heart and hope to die.”  It is sometimes reinforced by making a sign of the Cross over one’s chest.

Tradução: juro de pés juntos.