Looking for that perfect, simple costume that you can throw together last minute, but that will still reference an aspect of pop culture that's instantly recognizable to anyone who watches TV? Why not go with a Mr. Robot Halloween costume this year?
First you must look obvious and suspicious, so you're going to need a dark hoodie that screams "anarchist!" Pick a zip-up hoodie sweater that is of a thin, light style, rather than one of the baggier kinds. Get it as close as you can to the dark, nondescript ones that you see on the show. Make sure to always wear the hood flipped over your head and saunter around sullenly with your hands crammed into the pockets. The rest of your clothing should be jeans and a T-shirt, of course. What color T-Shirt? It doesn't really matter, since you should never unzip your hoodie.
The Hoodie & Clothing
Where is a hacker without all his hacker equipment? If you want to be authentic, you're going to need a black backpack with no obvious brand names on it, as nondescript as the hoodie. Just make sure the backpack looks full, but if you want to go the extra mile, you can fill it with actual "hacker stuff." Throw in a laptop with a destroy of Linux installed, an external hard drive, and a case of CD's.
Finally, you need to look like you've spent far too many nights staring at a glowing screen with nothing but excessive amounts of caffeine keeping you alive. Borrow your girlfriend's dark eyeshadow and paint some dark circles around your eyes. For extra authenticity, get minimal sleep a few days before Halloween so that you have genuine puffy eye-bags.