The inertia of motion is defined as an object's or body's inability to:

Inertia is the property of any object that prevents it from changing its state of being either at rest or in motion.

The inability of an object or body to change its state of motion on its own is referred to as inertia of motion. 

For example, 

Assume you are in a moving car when it abruptly stops, and all of the passengers fall forward. Because the lower part of the body in contact with the car is at rest, the upper part tends to remain in motion due to motion inertia.