"blood-red," late 14c. (late 12c. as a surname), from Old French sanguin (fem. sanguine), from Latin sanguineus "of blood," also "bloody, bloodthirsty," from sanguis (genitive sanguinis) "blood" (see sanguinary). Meaning "cheerful, hopeful, confident" first attested c.1500, because these qualities were thought in medieval physiology to spring from an excess of blood as one of the four humors. Also in Middle English as a noun, "type of red cloth" (early 14c.).
sanguine san·guine (sāng'gwĭn)
Of a healthy, reddish color; ruddy.
Cheerfully confident; optimistic.
Having blood as the dominant humor in terms of medieval physiology.
Archaic Having the temperament and ruddy complexion that was formerly thought to be characteristic of a person dominated by this humor; passionate.