There are several synonyms for the word "hard-heartedly", which is an adjective that refers to a lack of sympathy or compassion. Some of the closest synonyms include "inconsiderately", "callously", "unsympathetically", "coldly", and "indifferently". Other synonyms that convey a similar meaning but may have slightly different connotations are "ruthlessly", "unfeeling", "pitilessly", "insensitively", and "cruelly". These words are often used to describe someone who is indifferent to the suffering or needs of others and can be applied to a variety of situations, from a harsh boss at work to a derisive critic.