When talking about the Urban Dictionary ratings system, the opposite of the thumbs up. Where a thumbs up denotes a good definition and thumbs down denotes a bad one. There aren't many definitions out there whcih don't have at least one thumbs down. Whether or not people actually like a definition, they will give it a thumbs down, because they hate everybody, the world is unfair to them and they like to cut themselves.
Only assholes give definitions the thumbs down, unless it really sucks.