Isometric Wipers

Muscles worked

Filled = primary · Outlined = secondary

Source: Free Exercise DB (public domain)

Assume a push-up position, supporting your weight on your hands and toes while keeping your body straight. Your hands should be just outside of shoulder width. This will be your starting position.
Begin by shifting your body weight as far to one side as possible, allowing the elbow on that side to flex as you lower your body.
Reverse the motion by extending the flexed arm, pushing yourself up and then dropping to the other side.
Repeat for the desired number of repetitions.