How fast are you moving relative to your chair when sitting still?

When you are sitting still in your chair, your motion relative to the chair can be characterized by your speed and position. Since both you and the chair are stationary with respect to each other, you are not moving at all relative to the chair. This means there is no relative motion to measure; you and the chair share the same frame of reference.

In the context of physics, motion is always relative to an observer's frame of reference. In this scenario, since there is no change in position between you and the chair, the relative velocity is zero. Thus, you are not moving with respect to your chair.

While you might be moving at significant speeds relative to other frames of reference in the universe—such as the Earth's rotation or its orbit around the Sun—that is not relevant to the frame of reference defined by your interaction with the chair. Hence, being at rest in relation to the chair accurately describes your state when you are sitting still.