rotating the wrist with hand out straight so that the side of the tumb bangs on a desk. In a thumb thumb this is done twice in succession. This is used by teens taking their exams to signal to others around them they are finished and bored. This usually recquires another thumb thumb as a ressponse to show that it is understood and that the second person is also finished and bored.
Jay: (thumb thumb)
Tom: (thumb thumb)