Sanguine
Etymology
From Latin sanguineus, "of blood" (sanguis, "blood"). In medieval medicine, the body’s four humors set your temperament, and a person with dominant blood was "sanguine" — ruddy, warm, and buoyant. The physiology was wrong, but the personality word survived.
adjective · /SANG-gwin/
Cheerfully optimistic, especially in a difficult situation.
What it names
A confident, hopeful outlook that holds up under pressure — not naïve, but unrattled.
Example
Despite the setback, the founders stayed sanguine about shipping by spring.
Related idea
From the same humoral theory we also get melancholic, choleric, and phlegmatic.