A callus is a hard, thickened area of skin that develops usually from friction or irritation over time. Such a hardened area often leaves one less sensitive to the touch, so it's no surprise that the adjective callous, in addition to describing skin that is hard and thick, can also be used as a synonym for harsh or insensitive. Both callus and callous derive via Middle English from Latin. The figurative sense of callous entered English almost 300 years after the literal sense, and Robert Louis Stevenson used it aptly when he wrote, in Treasure Island, "But, indeed, from what I saw, all these buccaneers were as callous as the sea they sailed on."