A callous person is insensitive or emotionally hardened. If you laugh at your little sister while she's trying to show you her poetry, you're being callous.
Callous comes from the Latin root callum for hard skin. If you walk barefoot a lot, your feet will become calloused. We usually use callous in the metaphorical sense for emotionally hardened. If someone is unmoved by other people's problems, you might say he shows a callous indifference to human suffering.