When someone is being an extremely large downer, and is complaining, therefore ruining everyone elses good time as well. Can be elaborated on by adding statements such as "Stop eating all the fun" and "Why do you have to eat all the good-time that exists in this room?"
Can also happen when the person is not saying anything but just acting extremely mopey and not contributing to the conversation.
Girl: "This party is so lame. I'm so hungry and the music here sucks. We should leave. Aren't you hungry? Don't you think tonight was crappy?"
Guy: "God stop being such a downasaur and eating everyone else's good time!"