B.O. is a slang term used to describe body odor, which is an unpleasant smell that can come from someone's body. Some synonyms for B.O. include funk, stench, odor, and aroma. Other words that can be used instead of B.O. include foul smell, bad odor, and offensive scent. It's important to use polite and discreet language when discussing personal hygiene, so it's best to avoid using slang terms like B.O. Instead, try using more neutral and descriptive words that are less likely to offend or embarrass anyone.