Definition
An expression used to emphasize the truthfulness or sincerity of a statement or promise.
挺胸证明（所说皆真）
I promise you I won't tell anyone your secret, cross my heart and hope to die!
我向你保证我不会告诉任何人你的秘密，我发誓！
I will pay you back the money, cross my heart and hope to die, it's absolutely true.
我一定会把钱还给你，我发誓，绝对是真的。
Cross my heart and hope to die, I didn't take your wallet.
我发誓，我没有拿你的钱包。