Callus is a noun meaning a localized thickening of the skin, and a verb meaning to form a localized thickening of the skin. Callused means having many calluses. Callous is closely related to callus, but it’s figurative—that is, it doesn’t describe actual skin—and it is never a noun. As an adjective, it means toughened or unfeeling. As a verb, it means to make or become callous.
Examples
The most common error involving these words is the use of callous in place of callus—for example:
Exceptionally religious, he has a callous on his forehead from having prostrated himself in prayer so often. [NPR]
I grew accustomed to having mud under my fingernails, callouses on my hands and grass stains on my sneakers and my knees. [The Telegram]
Also questionable is the use of calloused where the nonparticipial callous would work—for example:
Now let’s turn from the body politic to the IRS and the calloused attitudes that years of severe budget restraints are fostering internally. [Nieman Watchdog]
I’m sure she is very desensitized and calloused to the repetitive violations to her dignity as a female. [letter to Montreal Gazette]
And here are some positive examples:
So is going barefoot a happy part of a carefree summer or a great way to step on bee, get Athlete’s Foot and develop calluses? [Atlanta Journal Constitution]
His hardcore ideology is too socially libertine for religious conservatives and too economically callous for crossover Democrats. [Telegraph]
By law, nail salons aren’t allowed to use callus shavers. [Buffalo News]
The case resonated with many Egyptians who had come to see the country’s police as a callous, heavy-handed force prone to abuses. [Washington Post]