That guy who, no matter what you have done, or are going to do, has done it better, bigger, louder, or to a greater extent, and will tell you about it. he cannot be second place. You can make something completely false up and he will top it. He never lies, lying implies that he doesn't believe it. If he didn't believe his own stories, no one would.
His friends growing up might have been mentally handicap and didn't realize the bullshit they were listening to. This explains why he thinks he is the best thing ever, and how he thinks he is getting away wit it.
It is best to avoid the 1upper at all costs. If contact is unavoidable, make up stories that no one (less the 1upper) would believe and watch the bullshit flow.
"I hate hanging out with Justin... he is such a 1upper. He kept telling everyone how his truck had more horsepower than Adam's and how it was so much better. Is he blind or retarded... his truck is 5 years older and his engine has been discontinued"
"My roommate takes the shortcut home, but Justin informed us that its not as fast when you drive an extended cab pickup 35mph through the roundabouts on the normal drive. That 1upping peace of shit... he couldn't take those roundabouts at 25mph, but he takes them at 35mph every day...Right"