The word bum has several meanings:

1. A person without a home or regular job who lives by begging or doing odd jobs. In old American stories, you might read about bums riding freight trains from town to town during the Great Depression, looking for work. The word is now considered disrespectful. More considerate terms include homeless person or person experiencing homelessness.
2. Something of poor quality or that doesn't work properly. If your knee gives you trouble, you might call it your bum knee. A flashlight with dying batteries could be called a bum flashlight. When something gives you a bum deal, you've been treated unfairly or gotten less than you deserved.
3. Your bottom or rear end. This is informal, casual language that kids often use instead of more formal words.
4. To get something from someone without paying, or by asking for favors repeatedly. If you forgot your lunch money, you might try to bum a few dollars from a friend. Someone might bum a pencil in class if they forgot theirs at home.

The verb phrase bum out means to disappoint or depress someone, like when rain ruins your outdoor plans and really bums you out.