Derp slap 

Violently thrashing your torso resulting in your arms flinging about and slapping unweary passersby. You must let the movement of your torso fling out your arm, it is not a derp slap if you move your arm yourself.

Maddie and Layne derp slapped each other in spanish class until someone got hurt.

Derp slap by Zesty Pepper August 25, 2011