Merriam-Webster dictionary defines a callus as “a thickening of or a hard thickened area, on skin or bark.”
For Jim Harbaugh, it's also one of his favorite football metaphors.
When Harbaugh was hired at Michigan football in December 2014, he took over a program that had finished 12-13 over the previous two seasons. The Wolverines didn’t know how to win more than they lost. They didn’t have a reliable starting quarterback, or offensive line, or really anything else to hang their hat on.