what is the origin of viscous drag on a body falling through a fluid?

Explanation

When a body falls through a viscous fluid, the layer of fluid incontact with the body moves along with the ball, with the velocity of ball, whereas the layer of fluid at large distance from the falling body remais at rest. As a result of , the different velocities. Every slow moving layer will tends to retard the adjoinging fast moving layer. This tussle of opposing the relative motion between different layers of fluid will produce viscous force (also called viscous drag) which tends to retard the motion of falling body. The value of viscous drag increases with the increaes in velocity of the body.