Someone's bum is the part of their body which they sit on.

A bum is a person who has no permanent home or job and who gets money by working occasionally or by asking people for money.

If someone refers to another person as a bum, they think that person is worthless or irresponsible.

Some people use bum to describe a situation that they find unpleasant or annoying.

If you bum something off someone, you ask them for it and they give it to you.

If you bum around, you go from place to place without any particular destination, either for enjoyment or because you have nothing else to do.

If the organizers of an event such as a concert want to put bums on seats, they want a lot of people to attend it.