(adjective)
not feeling or showing any emotion or sympathy for others, hard-hearted, cruel, and insensitive with a disregard for others;
e.g. He was callous in his interactions with them, so they felt that he didn't care about them.
(adjective)
thickened or hardened, with or as if with calluses (hardened patches of skin due to friction);
e.g. She had callous hands from working in the garden so much.
(verb)
to make something thickened or hardened, typically as a result of friction;
e.g. All of his work with heavy tools will callous his hands and make them rough.
(noun)
a hardened and/or thickened area of skin or tissue, typically resulting from being subjected to friction, i.e. a callus (alternate spelling);
e.g. She developed a callous on her heel from wearing her tight dance shoes.