sang-froid (n.)

also sangfroid, "presence of mind, coolness, mental composure," 1712, from French sang froid, literally "cool blood," from sang "blood" (from Latin sanguis; see sanguinary) + froid "cold" (from Latin frigidus; see frigid). "In the 17th c. the expression was in France often written erroneously sens froid, as if it contained sens "sense" instead of the homophonous sang "blood'." [OED].