Er ... how can I put it ... it's obviously not one to be taken literally! Becuse if you didn't have a heart ... well, you don't need to be a genius to work that one out.
Basically, it means, have mercy on me or take my feelings into account or stop being such a bully! I know the feeling ... it's not nice.
Use in Normal SpeechEdit
When dealing with Boss from Hell.
"What?? You expect me to work overtime on Friday? I might have plans then, you don't know! Have a heart!"