late 14c., "blood-red, of a blood-red color" (late 12c. as a surname), from Old French sanguin (fem. sanguine) and directly from Latin sanguineus "of blood," also "bloody, bloodthirsty," from sanguis (genitive sanguinis) "blood" (see sanguinary).

The meaning "cheerful, hopeful, vivacious, confident" is attested by c. 1500, because these qualities were thought in old medicine to spring from an excess or predominance of blood as one of the four humors. The sense of "of or pertaining to blood" (mid-15c.) is rare.

 Also in Middle English as a noun, a type of red cloth (early 14c.). It sometimes was used in the senses now going with sanguinary.

late 15c., "consanguinity, blood-relation," a sense now obsolete; see sanguine + -ity. Meaning "quality or character of being sanguine" is by 1737. 

late 14c., "proportioned mixture of elements," from Latin temperamentum "proper mixture, a mixing in due proportion," from temperare "to mix in due proportion, modify, blend; restrain oneself" (see temper (v.)). In old medicine, it meant a combination of qualities (hot, cold, moist, dry) that determined the nature of an organism; thus also "a combination of the four humors (sanguine, choleric, phlegmatic, and melancholic) that made up a person's characteristic disposition." General sense of "habit of mind, natural disposition" is from 1821.

1510s, "of the color of blood, of a deep red color;" 1640s, "of or pertaining to blood," from Latin sanguineus "of blood, bloody," from sanguin-, stem of sanguis (see sanguinary).