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n. - One who thrives on besting anyone else's joy or shame, with anecdotal evidence (stories) of how their own experiences/possessions are better in any and all ways.

The oneupsman cannot be reasoned with, only trapped in a state up "oneupsmania."
Guy 1: Man, today was so cool, I saw a rainbow.

Guy 2: Yeah, that's nice. I saw a double rainbow.

Caution: Do not attempt to one-up the oneupsman, or a parastoltic chain reaction of oneupsmanship can occur.

Guy 1: Oh really, because I saw it on a beach this morning when I found a clam with a pearl inside.

Guy 2: Oh man, that's so cool, because I was out there yesterday, and found enough pearls to make a necklace, while not ending my sentences in prepositions.

At this point, the oneupsman will descend into "oneupsmania" if provoked any further. The only way to win is to walk away, or attempt to trap them in a logical fallacy.

Guy 1: So, why did you leave any pearls out there? It seems odd that you missed one.

Guy 2: Well, there were so many, I didn't want to take them all, that would be selfish.

Guy 1: True, true. I think the longest string of pearls i've ever seen was about a foot long.

Guy 2: I've seen one two feet long.

Guy 1: Great, so why didn't you want to make one even longer?

Guy 2: ......<Head A'splode>
by Wilson W March 01, 2011
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