Luigi's Death Stare is an Internet meme created on YouTube by a YouTuber named "CZbwoi". The original video involves Waluigi getting hit by a green shell, then Luigi passes by Waluigi in slow motion, giving him a wicked stare. During the process, the song, "Ridin" by Chamillionaire, is playing in the background. Despite its popularity around on the Internet, the meme is only occasionally used in the Mario Kart 8 Community.
In Mario Kart 8, while driving near someone, Luigi can be seen looking at them. When the scene is played in slow motion, Luigi looks like he's giving that other racer a mean look (often called the "Death Stare"). The meme generally involves another racer getting hit by an item before Luigi makes the slow motion stare at them. Though this is used by other characters, Luigi is said to have the best stare to the other racers.
It's actually incredibly popular. Not just with Miiverse users and YouTube. Tom put it for his favorite picture and WAS EVEN MENTIONED ON THE NEWS! That's when you know it's a good joke to mess around with. Plus, it's obvious something like this is gonna be posted on various social networks.