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c.1400, "hardened," in the physical sense, from Latin callosus "thick-skinned," from callus, callum "hard skin" (see callus). The figurative sense of "unfeeling" appeared in English by 1670s. Related: Callously; callousness.
callous cal·lous (kāl'əs)
Of, relating to, or characteristic of a callus or callosity.