Whether saying "what" is considered rude or not depends on the context and tone in which it is used. In some situations, saying "what" can be perceived as rude if it is said with an aggressive or dismissive tone, indicating impatience or annoyance. It can come across as disrespectful or as if you are not paying attention to the conversation.
However, in other contexts, saying "what" may not be rude at all. For example, if someone didn't hear what was said or if they need clarification, a polite and neutral "what" can be appropriate.
To avoid any potential rudeness, it's often better to respond with a more complete and polite phrase, such as "I'm sorry, could you please repeat that?" or "Excuse me, I didn't catch that. Could you say it again?"
Overall, it's essential to consider the tone and the context in which you use "what" to ensure effective communication and to avoid unintentionally sounding rude.