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Anything that evokes a strong feeling of disgust or repulsion and/or a person who perpetually ruins good times. Butt Onions can also be used to describe your strong feelings as to what is "uncool" or just plain awful and unpleasant.
"Don't go in there, Joe just ripped a butt onion"

Allen : "Wanna ding dong and ditch?"
Burt : "Nah man we'll get caught"
Chris: "Burt, don't be such a butt onion."

"That movie was terrible!" "Yeah, totally butt onions."
by NzappaZap June 11, 2013