A portmanteau of the words Definition and Defamation.
This is the act of going to the trouble of inaccurately defining a word on Urbandictionary just to make a private joke at the expense of a member of your extended social circle, who, since you'll have to go to the trouble of dragging them to a computer, telling them the Urbandictiony url to type in, and then telling them which is the one word out of thousands that personally mentions the name of the person to get any kind of reaction out of them, thus negating the possible humour from the act of the joke, (since this is like having to stop and explain a joke after telling it), and thereby confusing the issue for everyone else in the world who is obviously not either you and your two sniggering sychopantic hanger-ons who think this kind of thing is even remotely interesting or clever.
High School Kid 1: "Ha ha, look, i just wrote wrote a description for 'Gay Loser' and put Cecil McWeedy's name there".
High School Kid 2: "OMG u r so kewl! Wait until he sees that Defination, this is the funniest joke ever"
(the next day)
High School Kid 1: "It's been a day. He's going to be soooo mad".
High School Kid 2: "Yeah, everyone in school is going to be calling him Gay Loser".
High School Kid 1: "He has to have seen it by now".
High School Kid 2: "Yeah, and everyone in the world knows he's a Gay Loser".
High School Kid 1: "You know what would be really funny? If we showed him!"
High School Kid 2: "I'll get URL up and you bring him over. Let's do it".
High School Kid 1: "OMG, this will be so funny".
High School Kid 2: "Wait... it's not there anymore".