Negative inertia

[Mt. Nixion]

An object with positive inertia resists acceleration. An inertialess object would not resist acceleration at all. What would an negative inertia object be like?

[Galileo]

That's when you throw a ball forward, but it ends up smacking you in the face.

[Sir_Deenicus]

I would not word it such since the concept of force is too slippery for the intuition. For example, how much force does it take to accelerate light? :)

But yeah, like Galileo said, an object with negative inertia also resists a change in its motion, simply in the opposite direction. Thus we find the amusing conclusion that placing a body with negative inertia in a (positive) gravitational field causes it to fall since the field pushes it away, causing it to come closer.