A question to the linguistically minded among you

[karohemd]

While watching Carnivále, the term geek came up at various times, describing one of the acts (Henry Scudder was one)

I looked it up and M-W online had the following definition:
"a carnival performer often billed as a wild man whose act usually includes biting the head off a live chicken or snake"
and of course the one we all know.
(The etymology is given as "probably from English dialect geek, geck fool, from Low German geck" but the German definition of Geck I know is a fop, a man in fancy clothes, overdrawn manners and often not very bright.)

I wonder how the first definition evolved into the second. Any ideas?