From the desk of Fiction’s father, Ideas:
Sanguine comes from the Latin Sanguinis, of or having to do with blood. In medieval medical practice, there were four humors of the body created by varying combinations of temperature and humidity (hot, cold, wet, dry), and personality depended upon the most prominent humor. The humors were bile (choleric), black bile (melancholic), phlegm (phlegmatic), and blood (sanguine).
However, the term “bloody” is normally used in a pessimistic way, while “sanguine” refers to an optimistic person.