Used in the same context, and with the same emotion as "WooHoo!" when you are celebrating/very satisfied with something. Originating from predictive text on mobile phones when woohoo is typed in.
People who are infected with Onion infection.
They can infect you by eating onion that's influenced by the Lords of Onion.
After they're infected, they turned in to a mindless creature and with time the onion seed blossom on their head.