Or in motion. To paraphrase Newton, inertia is the tendency of a body to resist a change in its state, whether it is at rest or moving at a uniform speed in a straight line.
Roughly speaking, because of inertia it will take the same amount of energy to stop a cannonball, as it does to fire it out of the cannon. Which is why they don't stop moving when they hit houses, cars, redcoats, and so forth.
“It’s been a long time since I took physics but as I remember it inertia was the tendency of a body at rest to stay at rest. Or in motion.”
I forgot that last part, thanks.