Something idle is not active. If your car is idling, it's running but not moving. If someone calls you idle, it either means they think you don't have enough to do or that you're just plain lazy.
Idle can also mean having no value or purpose: idle rumors are rumors that people make up when they're bored, but have no grounding in fact. As a verb, idle can also refer to a car engine that is running while the vehicle is not moving. The adjective descends from Middle English idel, from Old English īdel "empty."