Defined by the American Heritage Dictionary as:
n. Coolness and composure, especially in trying circumstances.
[French : sang, blood (from Old French, from Latin sanguis) + froid, cold (from Old French, from Vulgar Latin *frigidus, alteration of Latin fr?gidus)]
Defined by the Merriam-Webster Online Dictionary as:
n. self-possession or imperturbability especially under strain
Etymology: French sang-froid, literally, cold blood
Date: 1750