Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
sanguine, sanguinary (adjs.)
The Latin sanguis, “blood,” is behind both of these, but their meanings differ sharply. The medieval sanguine humor was dominated by the blood; hence a sanguine person was red-cheeked, vigorous, and cheerful, and so sanguine came to mean “cheerful, optimistic, confident.” Sanguinary, however, means “bloody, bloodthirsty, or bloodstained” and by extension “grim, cruel, murderous.”
