One of the mysteries of the English language finally explained.
A practically bloodless heart that has been temporarily bypassed and opened for examination or surgery. Usually attributive (frequently in form open-heart), designating or relating to such an operation.
1950s; earliest use found in Surgery. From open + heart [interjection, adverb].
open heart/ˌəʊp(ə)n ˈhɑːt/
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