Lacking mercy; incapable of pity.
The first mention in print in English of the term 'hard-hearted' is in Geoffrey Chaucer's 1374 translation of Consolation of Philosophy, the 6th century treatise by the Roman philosopher Anicius Boethius:
Ne no tere ne wette his face, but he was so hard-herted.
There are references to 'a hardened heart' in early Latin and Old English versions of the Bible and Chaucer may have been influenced by that. However, he does appear to be the first person to have used 'hard-hearted' in print.
The term reflects the mediaeval belief that the heart was the organ that controlled one's thoughts and feelings - there being no understanding of the functioning of the brain at that time. The belief was that the condition of the heart reflected the senses in a direct and literal way. We have retained several mediaeval expressions that we now see as entirely figurative but which were previously akin to a medical diagnosis:
The last on that list, whole-hearted, is atypical in that it is a 19th century term and derives from a different meaning of 'hearted', that is, 'courageous;spirited'.
The transition from literal to figurative meaning is matched by the transition in the spelling of the terms. Initially, the two words were usually written separately, then later as a hyphenated pair and finally as a single word. Someone with a 'light heart' was initially 'light hearted', later 'light-hearted' and more recently 'lighthearted' - for example:
John Palsgrave's dictionary Lesclarcissement de la Langue Francoyse, 1530: "Lyght herted or mery, alaigre."
William Cowper's poem The Task, 1785: "He whistles as he goes, light-hearted wretch, Cold and yet cheerful."
Wilkie Collins's novel Queen of Hearts, 1859: "Mrs. Knifton began to make jokes about it, in her lighthearted way."
The 15th century literal way of thinking (and spelling) was resurrected in the 1920s in the popular Tin-Pan Alley song Hard Hearted Hannah:
They call her Hard Hearted Hannah,
The vamp of Savannah,
The meanest gal in town;
Leather is tough, but Hannah's heart is tougher,
She's a gal who loves to see men suffer!
Of course, to be 'hearted' these days we just need a T-shirt. The 'I heart NY' message began being used in the USA in the 1980s and, in January 2011, the OED defined a new meaning of the verb heart as "To love; to be fond of: - originally with reference to logos using the symbol of a heart to denote the verb 'love'", which is as close as we can get to an acceptance that the symbol is now part of the language.
See also: the meaning and origin of 'half-hearted'.