Sanguine isn't really an unusual word. We see and hear it used fairly often, but it's sometimes a bit disconcerting to line up the way it's usually used, with the dictionary definition. Most of the time, we hear people using sanguine to mean cheerful or happy; sometimes in the inverse, as "I'm not sanguine about my chances at winning the lottery." The part that's disconcerting is that we also use sanguine to mean "bloody." The AHD definition includes both meanings:

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

Sanguine, and related words, entered English by way of Old French, sanguin, from Latin sanguis, sanguin-, meaning "blood." Somewhat confusingly, Modern English uses sanguine, in its third definition, the "cheerfully confident; optimistic" definition, but the same root also gives us "sanguinary," or "bloodthirsty." The reason for this seeming contradiction is embedded in medieval medicine.

In the middle ages, physicians believed that perfect health consisted in a balance between the four humors. The humors were four bodily fluids, blood, bile, black bile, and phlegm, which each governed not only physical characteristics, but emotional states and qualities. If someone had an overabundance of blood, so that it was the dominant humor, they were believed to have a cheerful disposition, one full of good will, hope, and given to readily falling in love. Such people were said to have a sanguine disposition. We no longer associate a sanguine state of mind with an excess of blood, but the word still bears its bloody heritage—as does sangria, the red-wine based cocktail.