past-participle adjective from harden (v.). Figurative sense of "unfeeling" is from late 14c.
Entries linking to hardened
harden n.
c. 1200, transitive, "make (something) hard," from hard (adj.) + -en (1). Intransitive meaning "to become hard" is late 14c. The earlier verb was simply hard, from Old English heardian. Related: Hardened; hardening.