To thrust ones arm in and out of another persons resting arm. It is usually done by boys with uncontrolable hormones, or someone who is simply bored. The victims are usually quiet, unsuspecting nerds.
A double arm rape is when one thruts both his arms in and out of both the resting arms of a victim.
Blake was sitting on the bench when Dennis sneaked up behind him and arm raped him.
Blake's arm has never been the same since.