XD is an abbreviation for Xtreme Donkeys. On the Internet XD is a common term that is used every time something funny is mentioned, telling the person who made the funny remark that they are not stupid and knew it was meant to be funny, but not good enough for a lol.
X-treme Donkeys is a video that has been passed around many times on the Internet. In the video donkeys do things so extreme that it would be inappropriate to mention them. Everyone on the Internet has seen this movie, and the use of XD is intended to remind people of how funny the movie was.
Every person on the Internet has seen Extreme Donkeys. If you haven't seen it, that's because you're stupid and not cool enough. Ask one of your friends for it. They will immediately claim to not know anything about Extreme Donkeys. They are lying. Such is the code of those who have seen the XD video. You must persist and ask them until they give it up. The Extreme Donkeys code states that you shall ask 510 times. (It is really 500 times, but this is in case the person with the video can't count well). If the person refuses to hand over the video, you should kill them, much as you would a mime.
There is also a variation of XD called XDD. XDD stands for Except for Dumb Donkeys, which is the opposite of XD. People also tend to add several excess "D"s. This is to emphasize how dumb the donkeys were. So saying "XDDD" would mean "Except for dumb, dumb, donkeys." This is again a reference to the video, but this variation expresses the opinion that the donkeys were extremely stupid.
There is also the definition of Extreme Diarrhea, whereby somebody produces liquid excrement to an extreme measure. Michael Bolton approves of this definition. See also tubgirl.
In 1687, the game company TwattyBollocks Fungames made the game Pokémon XD, which was based on the Extreme Donkeys video. However, the game turned out to have absolutely nothing to do with the video whatsoever, and was in fact exactly the same as their previous game Pokémon Colosseum, which was a racing game based on the Game Boy classic comic strip, Pokémon Stadium. Pokémon XD is available from all good retailers, and even some bad ones, and is only compatible with the Flipendo Qbox game system.
<sarcasm> XD is so important, this page should be in FAVORITES </sarcasm>
<rondelst> so then i say 2 the bartender, thats no cat, thats my wife! <ext799>XD <biscuitbrah>XD <5ndiner>XD <biscuitbrah> must be caturday <rondelst>XDD <worker> George, we nuked Iran off the face of the planet <George Bush> XD pwn <DAVE> I AM GAY <JESSE> XD <Bill Clinton> Okay, Hillary. Check this thang out! <Hillary Clinton> XDD <Cait> My BF dumped me last night =( <Mat> XD