A bump is an area that bulges out, like a pregnant woman's stomach. To bump someone is to knock into them.
A pimple is a little bump. A camel’s hump is a larger bump. There are speed bumps sticking up in the road to keep drivers from going too fast. When you bump someone, you hit them with your whole body. If the person in front of you stops walking and you're not paying attention, you might bump right into them. Bumping often happens by accident, unless of course you are doing "the bump" (a 1970's dance).