made hard; hardened.
insensitive; indifferent; unsympathetic: They have a callous attitude toward the sufferings of others.
having a callus; indurated, as parts of the skin exposed to friction.
verb (used with object), verb (used without object)
to make or become hard or callous.

1375–1425; late Middle English < Latin callōsus hard-skinned, tough, equivalent to call(um) tough skin, any hard substance + -ōsus -ous

callously, adverb
callousness, noun
uncallous, adjective
uncallously, adverb
uncallousness, noun

callous, callus.

1. hard. 2. inured, insensible, obtuse. See hard.

1. soft. 2. sensitive. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
callous (ˈkæləs)
1.  unfeeling; insensitive
2.  (of skin) hardened and thickened
3.  pathol to make or become callous
[C16: from Latin callōsus; see callus]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1400, "hardened," in the physical sense, from L. callosus "thick-skinned," from callus, callum "hard skin" (see callus). The figurative sense of "unfeeling" appeared in English by 1670s.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

callous cal·lous (kāl'əs)
Of, relating to, or characteristic of a callus or callosity.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Encyclopedia Britannica


in dermatology, small area of thickened skin, caused by continued friction, pressure, or other physical or chemical irritants. In response to mild but repeated injury, the cells of the epidermis, the outermost horny layer of the skin, become more active, giving rise to a localized increase in horny tissue on the surface of the skin. Calluses are most frequently seen on the hands and feet. They are usually yellowish white, flat, and painless. When a callus is conical in shape, penetrating into the deeper layer of the skin and causing pain when pressed, it is called a corn

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
We call a callous turncoat a "rat." Rats and mice, however, are
  giving scientists clues to the evolutionary origins of empathy.
People tend towards generosity after holding a warm cup of coffee, and are more
  callous after holding a cold drink.
Her anguished memories of painful treatment at the hands of callous and brutal
  doctors provide clues to her troubled psyche.
Other misadventures include demoralizing casting calls, conniving friends and a
  string of callous boyfriends.
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