Callus is a spit of contempt.
It usually means, of course, a thickening of the skin caused by continual rubbing or pressure.
(Let's stop for a moment to admire the miracle that is the human body, shall we...yep, good stuff.)
A callus can also be the extra strengthening tissue that forms around the mend in a broken bone (and I mean, how does it know to do that???)
Plants do the same sort of thing round any injury, too (which, okay, makes me feel slightly less wonderful.
A Sherlock Holmes would be able to divine my job from the middle finger of my right hand, because anyone who does a lot of handwriting will have an area of thickened skin on the thumb-side of their top finger joint.
You know, I've never noticed that before.
The word callous, meaning unfeeling or insensitive, is basically the same word: because, of course, if you're thick-skinned you don't feel much.
Word Not To Use Today: callus/callous. Callum is Latin for hard skin, and callus is a variant of that.