From Latin 'callosus' (thick-skinned) — emotional numbness as a metaphor of physical hardness, coined in Latin itself.
Showing a cruel disregard for others; having hardened, thickened skin.
From Latin 'callōsus' meaning 'thick-skinned, hard,' from 'callum' or 'callus' (hardened skin, a callus). The Latin noun derives from PIE *kal- (hard, rock-like). The semantic path ran: hard skin on the body → emotional hardness. Roman physicians used 'callum' clinically for skin hardened by friction or repeated pressure; Cicero extended it metaphorically to the mind