1. 1. Of the color of blood; red.
   2. Of a healthy reddish color; ruddy: a sanguine complexion.
   3. Having blood as the dominant humor in terms of medieval physiology.
   4. Having the temperament and ruddy complexion formerly thought to be characteristic of a person dominated by this humor; passionate.
2. Archaic
   1. Having blood as the dominant humor in terms of medieval physiology.
   2. Having the temperament and ruddy complexion formerly thought to be characteristic of a person dominated by this humor; passionate.
3. Cheerfully confident; optimistic.

Word History: The similarity in form between sanguine, "cheerfully optimistic," and sanguinary, "bloodthirsty," may prompt one to wonder how they have come to have such different meanings. The explanation lies in medieval physiology with its notion of the four humors or bodily fluids (blood, bile, phlegm, and black bile). The relative proportions of these fluids was thought to determine a person's temperament. If blood was the predominant humor, one had a ruddy face and a disposition marked by courage, hope, and a readiness to fall in love. Such a temperament was called sanguine, the Middle English ancestor of our word sanguine. The source of the Middle English word was Old French sanguin, itself from Latin sanguineus. Both the Old French and Latin words meant "bloody," "blood-colored," Old French sanguin having the sense "sanguine in temperament" as well. Latin sanguineus was in turn derived from sanguis, "blood," just as English sanguinary is. The English adjective sanguine, first recorded in Middle English before 1350, continues to refer to the cheerfulness and optimism that accompanied a sanguine temperament but no longer has any direct reference to medieval physiology.