1
general::
Be merciful, show pity; also, be reasonable. For example, Have a heart? I can't pay you back until next month, or Have a heart and stop your arguing now. This expression is often put as an imperative, as in the examples. [c. 1900] Also see HARDEN ONE'S HEART.
American Heritage Idioms
2
general::
Phrase(s): have a heart
Fig. to be compassionate; to be generous and forgiving; to have an especially compassionate heart. • Oh, have a heart! Give me some help! • If Anne had a heart, she’d volunteer to help us on the charity drive.
McGrawhill's American Idioms And Phrasal Verbs