1. To annoy someone with a strange word
2. To let others know you don't care what they're saying
3. To be funny
While Kellen played video games with his friend, the other kids online were being stupid so Kellen said "peeoouup" to show them he couldn't care less what they were saying.
Johns girlfriend was talking to her friends so he snuck up behind and said "peeoouup" to be bothersome