If you're unmotivated, you have no drive. The guy who spends all day on the couch, living off the check that he got when his dog discovered dinosaur bones buried in the backyard? Clearly unmotivated.
Unmotivated, motivated, motion, and automotive — all these words are related to the Latin word motor, which means "movement." If you're unmotivated, it means you lack the internal motor that would make you get off the couch and do stuff.