I couldn't find a good etymology, but here's some early usage according
the the OED:
<<
geek (ik). [Var. GECK sb.1] U.S. slang. (See quots. 1954 and 1961).
[1876: see GECK sb.1]. 1916 Wells Fargo Messenger Oct. 29/2 A new Wells
agent struck our town the other week, and say - you never saw a more
enthusiastic geek! 1954 WEBSTER Add., Geek, a carnival `wild man' whose
act usually includes biting the head off a live chicken or snake. 1961
Times Lit. Suppl. 27 Jan. 62/2 He picks up waitress, a simple girl, and
enslaves a `geek', a dumb sideshow stooge whose daily routine consists of
being exhibited in a pit which he has to dig for himself.
>
-- Doug