A poser onion is basically another word for "poser". It was derived from the Veggie Burger bags from A&W. The bag features three vegetables all smiling. One day, while partaking of this burger, it was discovered that the onion looked somewhat suspicious, and out of place looking the way he did. It was decided that he was a "poser onion", and anyone who was a poser, is also a poser onion.
Most emo kids are poser onions.