Well the term "Geek" is referred to a freak who will basically eat anything (live snakes, rats, what ever) But the term geek magic is loosely based off of this definition. Geek Magic is basically and form of magic that involves a freak show style (thread for example is a geek trick)
No. Way back before geek came to mean what you commonly hear in high school, it was a term applied to circus freaks. The term referred to someone who would chase a chicken around only to bite the head off, who acted as sort of an opener for other freaks. However, it has come to also include also sorts of circus sideshow beings - Siamese twins, the human blockhead, sword swallower, etc., they could all be called Geeks.
As such, Geek magic tends to be grotesque, gory, and bizarre. Saw and Thread are examples. The right presentation can make Wounded or even Control an example of Geek magic.
Geek magic is any sideshow magic trick type effect that is accomplished with magical means.
Sideshow is something you shouldn't mix in with geek magic so easily, half of what you see in sideshow is real, the other half is fake. It's trying to determine which is real and which is not.