Definition
An expression used to emphasize the truthfulness or sincerity of a statement or promise.
挺胸證明（所說皆真）
I promise you I won't tell anyone your secret, cross my heart and hope to die!
我向你保證我不會告訴任何人你的秘密，我發誓！
I will pay you back the money, cross my heart and hope to die, it's absolutely true.
我一定會把錢還給你，我發誓，絕對是真的。
Cross my heart and hope to die, I didn't take your wallet.
我發誓，我沒有拿你的錢包。