What does it mean by "heart is borrowed"?
Native speaker’s answer
Rebecca
My guess is that it means her heart may have belonged to (she gave her heart away) others in the past, hence why she says it was "borrowed" here. A similar expression used to express that one loves another is to say that one's "heart belongs to another." "My heart's been borrowed" probably means something similar, that her heart has belonged to others in the past.