adverb (or adjective)
1. : footloose and wandering around the country in the manner of a tramp : in the manner or role of a vagrant
he would go back on the bum awhile and try some other city — James Jones
travel across country on the bum
2. : sponging , cadging
when his money ran out he always came to me on the bum
3. : in poor order or condition
my wrists are a bit on the bum — Sinclair Lewis
: in a state of depression
the old days in the States before everything went on the bum there — Ernest Hemingway