(informal.) noun 1. a person’s bottom ○ He just sits on his bum all day, doing nothing. 2. US a person who sits around doing nothing ○ Can’t you bums find something to do? 3. a person who is very keen on something ○ a ski bum ■ verb □ to bum something off someone to ask someone for something ○ He always trying to bum cigarettes off his friends. ○ Can I bum a cigarette?