Put (one's) hand on (one's) heart verb phrase
To state or promise very strongly that something is completely truthful, sincere or trustworthy
Jimmy, trust me, I have not done it. - I dare you to put your hand on your heart and say that again!
He puts his hand on his heart, claiming that he has never cheated on her.
Nobody trusts Halley anymore because even after putting her hand on her heart, she can still break the promise and lose other people's trust.
To say bad words with a whisper.
The verb "put" should be conjugated according to its tense.