A person who always has a better story. If you killed six goats, he or she killed 23. Most of the time they will get angry because you told a story about ONLY killing 6 goats.
Person 1: Wow, this is a nice view. How much land do you own?
Person 2: Umm, I think about 69 acres.
Person 3(One upper): YOU own 69 acres!?!?!?! MY UNCLE OWNS 178!!
A one-upper is someone who always has to be bigger and better than everyone else. If you say you are drunk, they exclaim with how much more they have drank than you.
Although Matt may seem mediocre, he is actually a one-upper; He must always top your story with one of his own.