Uncivilized means wild and barbaric, although people also use it to mean impolite. The uncivilized three-year-olds ate lasagna with their hands and then chased the cat around the house before their parents intervened.
If a person is civilized, they behave politely. When an entire group or culture is described as civilized, it means they are socially, culturally, or technologically advanced. When you add the prefix un- ("not") you get the opposite: "That town is so uncivilized, they don't even get cellphone reception." And your little brother, burping in the back of the car while he kicks your seat from behind? Definitely uncivilized.