What does Yapper mean?

Someone who talks excessively

Yapper is typically used in informal, relaxed settings where slang naturally thrives—think group chats, casual hangouts, online forums, and social media banter. It can appear in a friend’s text message complaining about a roommate who won’t stop talking, or in a Reddit comment section describing that one user who writes multi-paragraph responses to every post. You might whisper it to a friend at a party when someone’s monologue is running too long. Because it’s a slang term, it’s less common in professional or academic environments. Instead, it finds its niche in everyday social circles where people feel comfortable calling each other out with a bit of humor.

People say "Yapper" to express annoyance or frustration with someone's chattiness.