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.
updated on December 10, 2021