Self-possession and coolness under strain.
Self-possession and coolness under strain — steadiness of mind and nerve in the face of danger or pressure.
Among the calm words, sangfroid points specifically to marked coolness and steadiness when the strain is real — the unflinching nerve of someone facing actual danger or high stakes (“ice in the veins”), carried with an admiring tone. Equanimity names a settled cast of mind seldom shaken; composure names agitation mastered by deliberate effort; sangfroid puts the spotlight on cool nerve in the perilous moment, more vivid than plain calmness.
From French sang froid, literally “cold blood” (sang “blood” + froid “cold”). In 17th-c. France often miswritten sens froid, as if from sens “sense.”
Borrowed into English in the early-to-mid 1700s (etymonline 1712; Merriam-Webster 1750) and stable since. English already used “cold-blooded” pejoratively for the ruthless; sangfroid was borrowed to give “ice in the veins” a positive spin — composure, not cruelty.