When I was growing up we were taught that people without jobs living in the streets were called bums or tramps. When I moved to Seattle for university these same people were referred to as transients. In Australia they are referred to as deros, short for derelicts.

Bum simply means rear end here. I know of a joke about a Lord Bothamleigh going off to visit Lord Cholomondeley. When they met Lord Cholomondeley told Lord Bothamleigh, "Please, do call me Lord Chumly." To which Lord Bothamleigh replied, "Then you must call me Lord Bumley."