callus-, callous- +

(Latin: hardened skin, thick skin; a knob)

Cross references directly, or indirectly, involving the "skin": chorio-; cicatri- (scar); cori-; cuti-; hymen-; lepido- (scab, scale); papulo- (pimple); psoro- (itch, mange); pustu- (blister, pimple); rhytid- (wrinkle); scabio- (mange, itchy); sebo- (grease, oil).