Selfies have been around for quite a while, but with the popularity of social media (in particular Instagram) they've become ubiquitous in our culture. There's even mobile apps dedicated solely to selfies. If that and your news feed isn't proof enough, just check with the Oxford Dictionary.
So, if you're looking for a Halloween costume that's both easy and timely, grab a piece of posterboard and turn yourself into a selfie.
Just use markers, paint or colored paper to make the blue Instagram banner across the top, cut out a square for your portrait, and add some other details like your username, some likes, and a few clever hashtags. You can even add a transparent color sheet of plastic as a filter.
And if the costume itself isn't meta enough for you, post a photo of yourself wearing it to Instagram à la Beyoncé!
It may not be the most elaborate costume, but everyone will get it and it's guaranteed to get you a few laughs. If all else fails, you can always just be Instagram.