Like "queer" or "bitch," this term for a wandering mendicant has long since been re-appropriated, as in the song, "Hallelujah, I'm a Bum." As opposed to the guy who sits in the same spot every day asking for a hand-out, the bum [from the German for "saunter"] roams freely throughout the city, the country, the planet: He is king of the road. See: BEGGAR, LOAF, SAUNTER.
In the United States, a hobo or vagrant. In the United Kingdom, the bottom, rear end, or "sit-spot." The traditional place to apply a slippering to the naughty victim.