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"sideshow freak," 1916, U.S. carnival and circus slang, perhaps a variant of geck "a fool, dupe, simpleton" (1510s), apparently from Low German geck, from an imitative verb found in North Sea Germanic and Scandinavian meaning "to croak, cackle," and also "to mock, cheat." The modern form and the popular use with reference to circus sideshow "wild men" is from 1946, in William Lindsay Gresham's novel "Nightmare Alley" (made into a film in 1947 starring Tyrone Power).
"An ordinary geek doesn't actually eat snakes, just bites off chunks of 'em, chicken heads and rats." [Arthur H. Lewis, "Carnival," 1970]By c.1983, used in teenager slang in reference to peers who lacked social graces but were obsessed with new technology and computers (e.g. the Anthony Michael Hall character in 1984's "Sixteen Candles").
geek out vi. To temporarily enter techno-nerd mode while in a non-hackish context, for example at parties held near computer equipment. [Eric S. Raymond, "The New Hacker's Dictionary," 1996]
[origin unknown; perhaps related to British dialect geck, geke, ''fool''; according to David Maurer, ''said to have originated with a man named Wagner of Charleston, WV, whose hideous snake-eating act made him famous'']
To temporarily enter techno-nerd mode while in a non-hackish context, for example at parties held near computer equipment. Especially used when you need to do or say something highly technical and don't have time to explain: "Pardon me while I geek out for a moment."
See computer geek, propeller head.