Gone but not forgotten

Quintessential: Refers to the fifth element, the quintessence, that holds everything together. It was once believed that everything was a combination of earth, air, fire, and water, and held together by quintessence.
Good humor: Refers to the notion that there are four humors in the body that are responsible for his health and stuff.
   full of bile: Supposedly, if you're angry all the time, you've got too much bile.
   phlegmatic: If you're sluggish, you've got too much phlegm.
Blue blood: In Spain, a blue blood used to be somebody with no Moorish or Jewish ancestry. According to the Online Etymology Dictionary, a reference to the fact that you can see veins through the skin of pale people. Maybe not so much disproved as secretly racist.
He's compensating: You might say this about a guy with a fancy car or a giant gun... it comes from Freud's notion that people with big cars or other displays of manliness were compensating for small manly bits. Which, we now know, is pretty much crap. But we've been over that before.
Hair of the dog: According to The Phrase Finder, back in the day, folks believed if you got bitten by a rabid dog, you could put that dog's hair on the wound and not get rabies. I don't know if this works any better than the metaphorical hair of the dog, but it certainly can't work any less.
Lunatic: From the belief that the moon made people crazy.