Is it ensure, insure, or assure?
"hardened skin," 1560s, from Latin callus, variant of callum "hard skin," related to callere "be hard," from PIE root *kal- "hard" (cf. Sanskrit kalika "bud," Old Irish calath "hard," Old Church Slavonic kaliti "to cool, harden").
callus cal·lus (kāl'əs)
n. pl. cal·lus·es
The hard bony tissue that develops around the ends of a fractured bone during healing.