A guy who always has to one-up everything anyone says or does. If you say you ran a 5 minute mile, he ran a 4 minute mile. If you say you went swimming this weekend, he'll tell you he's a certified lifeguard and swims every weekend. If you say you made coffee this morning, one-up guy will tell you that he grew, harvested, roasted, ground, and brewed his coffee. Usually the one-up explanations are long-winded, boring, and self-serving. Most of the one-up explanations are probably lies.
"I was telling him about my ski trip to Taos. One-up guy over there spent 20 minutes talking about how he used to be a ski instructor in Taos. I hate one-up guy."