According to urbandictionary:
An eccentric upper body dance move that involves the shaking of the upper torso and shoulders. The Harlem Shake originated on 125th and Lexington where gang bangers would shoot unsuspecting cripples with crutches. The staggering fall of these cripples created a signature move, adapted by gang-affiliates and introduced at local parties. Upon witnessing the newly created “Harlem Shake,” a corporate thug under the Bad Boy Entertainment payroll reenacted and sold this dance to famous choreographer Laurie Ann Gibson who introduced this dance with the video premier “Let’s Get It” by P. Diddy featuring G-Dep and Black Rob. The phenomenon sweeping white neighborhoods and Italian nightclubs across the country was born… the Harlem Shake.