|I'm not even going to pretend I'm not adorable.|
Quite a while ago, one of my friends wondered aloud why the word ginger is used to describe red heads. The friend pointed out that ginger is brown on the outside and yellow on the inside. Neither of which, the astute observer will notice, is red.
So according to the Oxford English Dictionary (okay, according to someone at this blog, quoting the Oxford English Dictionary, to which I don't have access), ginger was first used as a derogatory term to describe red-haired people in England in the 16th century.
My first thought was that it referred to gingerbread, which is sort of reddish in color. A lot of other folks seem to think the same, though it's kind of a reach. There are many things far redder than gingerbread. You'd think they'd more likely call us tomatoes or foxes or light districts.
Others say that the thing we call ginger today isn't the same as the thing they called ginger back when the term was born. According to ask.metafilter.com, ginger used to be used to mean mango ginger, a spice in the tumeric family, which is red and renders a reddish orange dye. I have no idea whether this is a reliable source, but the explanation is so random and boring I'm not sure why anyone would make it up.
As for the idea that gingers have no souls, I had assumed that the idea came from an episode of South Park titled Ginger Kids, in which Cartman delivers an impassioned speech about the evils of my kind, stating that gingers "are disgusting, inhuman, unable to survive in sunlight, and have no souls; all because of a condition called "Gingervitis." While it's certainly true that the South Park episode resurrected the myth in popular culture, it turns out that the idea goes back a much longer way.
Red hair has been considered bad across most cultures, according to the article Judas' Red Hair and the Jews found on the Jewish Heritage Online Magazine. That article points out that Judas was supposed to have red hair. There's evidence that the Greeks and Romans considered redheads undesirable. The god Thor was supposed to have had red hair, and Christians sort of merged Thor with Satan at some point (oddly, our image Santa Claus is partially influenced by the god Odin, Thor's dad). In mythology, vampires and witches were said to have red hair, and red hair is said to be associated with sexual prowess (to which I can attest).
The Jewish Heritage article gives a simpler opinion. That red hair has traditionally been considered bad because it is rare, and people fear what is different - the same reason left-handedness was once considered evil.
Update: the fabulous Jenna posted in the comments that there is such a thing as a ginger flower which is, in fact, bright red. Mystery solved?