Zoë: You sanguine about the kinda reception we're apt to receive on an Alliance ship, Captain? Mal: Absolutely. What's "sanguine" mean? Zoë: Sanguine. Hopeful. Plus, point of interest, it also means "bloody". Mal: Well, that pretty much covers all the options, don't it?
Came into English in the early 14th century, from the old French word, sanguin, which meant a type of red cloth.
Since the 15th century, it has also meant cheerful, hopeful, or confident. These qualities were thought in medieval time to come from an excess of blood, as one of the four humours in the human body.