cross my heart (and hope to die)
A vow that one is being truthful. Billy's the one who broke the cookie jar—cross my heart and hope to die! A: "Did you take that money?" B: "No, cross my heart!"
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
cross one's heart (and hope to die)
Fig. a phrase said to pledge or vow that the truth is being told. It's true, cross my heart and hope to die. It's really true—cross my heart.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.