Dictionary.com (again!) gives an interesting peek at the origin of this word, and that of some others...
"Our Living Language: Our word geek is now chiefly associated with student and computer slang; one probably thinks first of a computer geek. In origin, however, it is one of the words American English borrowed from the vocabulary of the circus, which was a much more significant source of entertainment in the United States in the 19th and early 20th century than it is now. Large numbers of traveling circuses left a cultural legacy in various and sometimes unexpected ways. For example, Superman and other comic book superheroes owe much of their look to circus acrobats, who were similarly costumed in capes and tights. The circus sideshow is the source of the word geek, “a performer who engaged in bizarre acts, such as biting the head off a live chicken.” We also owe the word ballyhoo to the circus; its ultimate origin is unknown, but in the late 1800s it referred to a flamboyant free musical performance conducted outside a circus with the goal of luring customers to buy tickets to the inside shows. Other words and expressions with circus origins include bandwagon (coined by P.T. Barnum in 1855) and Siamese twin."
Source: The American Heritage® Dictionary of the English Language, Fourth Edition
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