A big mouth hippo is a person who says something is wrong one day but then goes and does that exact same thing or has already done it in the past. Calling someone a hippo is like saying look who's talking. Comes from the word hypocrite, but might also be a reference to the really big mouth of a hippo when it opens wide.
1. A large amphibious land animal, looking like a fat, gray, leathery horse. They have a huge mouth with missing teeth and a foul temper. Do not approach or feed.
2. A fat woman dressing up in clothes several sizes to small. Do not approach of feed.